elfutils: fix gcc 8.0+ multistatement macros warning/error

elfutils: fix gcc 8.0+ multistatement macros warning/error

GCC 8.0+ <https://gcc.gnu.org/gcc-8/changes.html> introduces a new
warning about unsafe macros expanding to multiple statements used
as a body of a statement such as if, else, while, switch, or for.

In combination with -Werror this can cause the compilation to fail:

|In file included from xmalloc.c:37:
|xmalloc.c: In function 'xmalloc':
|system.h:39:2: error: macro expands to multiple statements [-Werror=multistatement-macros]
|  fflush(stdout); \
|  ^~~~~~
|xmalloc.c:52:5: note: in expansion of macro 'error'
|     error (EXIT_FAILURE, 0, _("memory exhausted"));
|     ^~~~~
|xmalloc.c:51:3: note: some parts of macro expansion are not guarded by this 'if' clause
|   if (p == NULL)
|   ^~

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>