tools/zstd: enable position independent code compilation

tools/zstd: enable position independent code compilation

Currently, trying to compile LLVM-BPF will fail with:
[2225/3517] Linking CXX shared library lib/libLLVM-15.so
FAILED: lib/libLLVM-15.so
/usr/bin/ld: staging_dir/host/lib/libzstd.a(zstd_common.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

So, to fix it enable PIC for the host ZSTD.

Fixes: #15247
Signed-off-by: Bryan Roessler <bryanroessler@gmail.com>