thread/Name: include stdio.h if HAVE_PRCTL

Caused a build failure with uClibc because snprintf() was not
available.

src/thread/Name.hxx

@@ -23,7 +23,6 @@
 #if defined(HAVE_PTHREAD_SETNAME_NP) && !defined(__NetBSD__)
 #  define HAVE_THREAD_NAME
 #  include <pthread.h>
-#  include <stdio.h>
 #elif defined(HAVE_PRCTL)
 #  include <sys/prctl.h>
 #  ifdef PR_SET_NAME
@@ -31,6 +30,10 @@
 #  endif
 #endif
 
+#ifdef HAVE_THREAD_NAME
+#  include <stdio.h>
+#endif
+
 static inline void
 SetThreadName(const char *name)
 {