libbpf: Install skel_internal.h header used from light skeletons

Light skeleton code assumes skel_internal.h header to be installed system-wide
by libbpf package. Make sure it is actually installed.

Fixes: 67234743 ("libbpf: Generate loader program out of BPF ELF file.")
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210603004026.2698513-4-andrii@kernel.org

tools/lib/bpf/Makefile

@@ -225,7 +225,7 @@ install_lib: all_cmd
 
 INSTALL_HEADERS = bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h xsk.h \
 		  bpf_helpers.h $(BPF_HELPER_DEFS) bpf_tracing.h	     \
-		  bpf_endian.h bpf_core_read.h
+		  bpf_endian.h bpf_core_read.h skel_internal.h
 
 install_headers: $(BPF_HELPER_DEFS)
 	$(call QUIET_INSTALL, headers)					     \