Fix gen_stubs with out-of-tree build

We need to prefix with $(top_srcdir).

Author: nikic

build/Makefile.global

@@ -143,7 +143,7 @@ prof-use:
 	CCACHE_DISABLE=1 $(MAKE) PROF_FLAGS=-fprofile-use all
 
 %_arginfo.h: %.stub.php
-	if type php >/dev/null 2>/dev/null; then php scripts/dev/gen_stub.php $<; fi
+	if type php >/dev/null 2>/dev/null; then php $(top_srcdir)/scripts/dev/gen_stub.php $<; fi
 
 # As we don't track includes, this is just a heuristic
 %.c: %_arginfo.h