perf tools: No need to include namespaces.h in util.h

The only thing that is needed there is a forward declaration for 'struct
nsinfo', so disentanble this, which in turns allows built-in clang
builds, i.e. 'make LIBCLANGLLVM=1 -C tools/perf'.

Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Naveen N. Rao <[e-mail]>
Cc: Sandipan Das <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/util.h

···
 #include <stdlib.h>
 #include <stdarg.h>
 #include <linux/compiler.h>
-#include <linux/types.h>
-#include "namespaces.h"
+#include <sys/types.h>
 
 /* General helper functions */
 void usage(const char *err) __noreturn;
···
 #define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
 
 struct dirent;
+struct nsinfo;
 struct strlist;
 
 int mkdir_p(char *path, mode_t mode);