samples/bpf: Fix a compilation error with fallthrough marking

Compiling samples/bpf hits an error related to fallthrough marking.
    ...
    CC  samples/bpf/hbm.o
  samples/bpf/hbm.c: In function ‘main’:
  samples/bpf/hbm.c:486:4: error: ‘fallthrough’ undeclared (first use in this function)
      fallthrough;
      ^~~~~~~~~~~

The "fallthrough" is not defined under tools/include directory.
Rather, it is "__fallthrough" is defined in linux/compiler.h.
Including "linux/compiler.h" and using "__fallthrough" fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201006043427.1891805-1-yhs@fb.com

samples/bpf/hbm.c

@@ -40,6 +40,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/unistd.h>
+#include <linux/compiler.h>
 
 #include <linux/bpf.h>
 #include <bpf/bpf.h>
@@ -483,7 +484,7 @@ int main(int argc, char **argv)
 					"Option -%c requires an argument.\n\n",
 					optopt);
 		case 'h':
-			fallthrough;
+			__fallthrough;
 		default:
 			Usage();
 			return 0;