libbpf: Fix libbpf_common.h when installing libbpf through 'make install'

This fixes two issues with the newly introduced libbpf_common.h file:

- The header failed to include <string.h> for the definition of memset()
- The new file was not included in the install_headers rule in the Makefile

Both of these issues cause breakage when installing libbpf with 'make
install' and trying to use it in applications.

Fixes: 544402d4b493 ("libbpf: Extract common user-facing helpers")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191217112810.768078-1-toke@redhat.com

src/libbpf_common.h

@@ -9,6 +9,8 @@
 #ifndef __LIBBPF_LIBBPF_COMMON_H
 #define __LIBBPF_LIBBPF_COMMON_H
 
+#include <string.h>
+
 #ifndef LIBBPF_API
 #define LIBBPF_API __attribute__((visibility("default")))
 #endif