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

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: 6bcd39d3 ("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

tools/testing/selftests/bpf/prog_tests/core_reloc.c

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
 #include <test_progs.h>
 #include "progs/core_reloc_types.h"
 #include "bpf_testmod/bpf_testmod.h"