libapparamor: Define a portable version of gnu basename

Since musl 1.2.5, basename(3) prototype is only provided in libgen.h
(as mandated by POSIX) and not in strings.h. Also there is a major
difference between the gnu basename and the one defined in libgen.h,
the latter modify the argument string making them incompatible.

Fix this by defining a portable version of basename using strchr.

libraries/libapparmor/testsuite/test_multi.c

@@ -1,5 +1,3 @@
-#define _GNU_SOURCE /* for glibc's basename version */
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -7,6 +5,12 @@
 
 #include <aalogparse.h>
 
+static const char *basename(const char *path)
+{
+	const char *p = strrchr(path, '/');
+	return p ? p + 1 : path;
+}
+
 int print_results(aa_log_record *record);
 
 int main(int argc, char **argv)