selftests/bpf: Fix compiling core_reloc.c with musl-libc

selftests/bpf: Fix compiling core_reloc.c with musl-libc

[ Upstream commit debfa4f628f271f72933bf38d581cc53cfe1def5 ]

The type 'loff_t' is a GNU extension and not exposed by the musl 'fcntl.h'
header unless _GNU_SOURCE is defined. Add this definition to fix errors
seen compiling for mips64el/musl-libc:

  In file included from tools/testing/selftests/bpf/prog_tests/core_reloc.c:4:
  ./bpf_testmod/bpf_testmod.h:10:9: error: unknown type name 'loff_t'
     10 |         loff_t off;
        |         ^~~~~~
  ./bpf_testmod/bpf_testmod.h:16:9: error: unknown type name 'loff_t'
     16 |         loff_t off;
        |         ^~~~~~

Fixes: 6bcd39d366b6 ("selftests/bpf: Add CO-RE relocs selftest relying on kernel module BTF")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/11c3af75a7eb6bcb7ad9acfae6a6f470c572eb82.1721713597.git.tony.ambardar@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>