lttng-tools: musl compile fixes

lttng-tools: musl compile fixes

Add two patches to address three distinct build problems spotted by our
build bots when compiling lttng-tools:

1) unconditional use of `__GLIBC_PREREQ`

On musl based toolchains there is no such macro defined, leading to the
following preprocessor error:

      CC       compat-epoll.lo
    In file included from compat-epoll.c:33:0:
    poll.h:76:19: error: missing binary operator before token "("
     #if __GLIBC_PREREQ(2, 9)

2) undeclared `mode_t` type

On musl based toolchains the `mode_t` type is not implicitely defined through
other includes, leading to the following compile error:

      CC       hashtable.lo
    In file included from ../../../src/common/common.h:24:0,
                     from hashtable.c:24:
    ../../../src/common/runas.h:25:46: error: unknown type name 'mode_t'
     int run_as_mkdir_recursive(const char *path, mode_t mode, uid_t uid, gid_t gid);
                                                  ^
    ../../../src/common/runas.h:26:36: error: unknown type name 'mode_t'
     int run_as_mkdir(const char *path, mode_t mode, uid_t uid, gid_t gid);
                                        ^
    ../../../src/common/runas.h:27:46: error: unknown type name 'mode_t'
     int run_as_open(const char *path, int flags, mode_t mode, uid_t uid, gid_t gid);
                                                  ^

3) multiple definitions

The header files declare several `const char *` pointers which are initialized
in various `*.c` files later on. Due to a missing `extern` declaration in the
header, the final linking of the executables fails with errors such as:

      CCLD     lttng
    ../../../src/common/.libs/libcommon.a(mi-lttng.o):(.data.rel.ro.local+0x0): multiple definition of `mi_lttng_element_snapshots'
    commands/enable_events.o:(.bss+0x18): first defined here
    collect2: error: ld returned 1 exit status

This commits addresses these issues with two patches, `100-musl-compat.patch`
fixes issue 1 by declaring a fallback dummy declaration of `__GLIBC_PREREQ` and
issue 2 by explicitely including `sys/stat.h` which provides `mode_t` according
to POSIX.

The second patch, `200-use-extern.patch` declares all char pointers in the
header file as `extern`, fixing the observed linker errors.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>