libbpf, hashmap: Remove unused #include

Remove #include of libbpf_internal.h that is unused.

Discussed in this thread:
https://lore.kernel.org/lkml/CAEf4BzZRmiEds_8R8g4vaAeWvJzPb4xYLnpF0X2VNY8oTzkphQ@mail.gmail.com/Signed-off-by: NIan Rogers <irogers@google.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200515165007.217120-3-irogers@google.com

tools/lib/bpf/hashmap.h

@@ -15,7 +15,6 @@
 #else
 #include <bits/reg.h>
 #endif
-#include "libbpf_internal.h"
 
 static inline size_t hash_bits(size_t h, int bits)
 {