tools/bpf: add missing strings.h include

Few files in libbpf are using bzero() function (defined in strings.h header), but
don't include corresponding header. When libbpf is added as a dependency to pahole,
this undeterministically causes warnings on some machines:

bpf.c:225:2: warning: implicit declaration of function 'bzero' [-Wimplicit-function-declaration]
  bzero(&attr, sizeof(attr));
    ^~~~~
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf.c

@@ -22,6 +22,7 @@
  */
 
 #include <stdlib.h>
+#include <strings.h>
 #include <memory.h>
 #include <unistd.h>
 #include <asm/unistd.h>

tools/lib/bpf/btf.c

@@ -4,6 +4,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <unistd.h>
 #include <errno.h>
 #include <linux/err.h>

tools/lib/bpf/libbpf.c

@@ -18,6 +18,7 @@
 #include <libgen.h>
 #include <inttypes.h>
 #include <string.h>
+#include <strings.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>