Fixed "make tests" build without preceding "make".

Currently almost all Unit object files depends on generated nxt_version.h.
This patch adds missing dependence and fixes running make with multiple
jobs.

This closes #318 issue on GitHub.

auto/modules/php

@@ -203,7 +203,7 @@ for nxt_src in $NXT_PHP_MODULE_SRCS; do
 
     cat << END >> $NXT_MAKEFILE
 
-$NXT_BUILD_DIR/$nxt_obj:	$nxt_src
+$NXT_BUILD_DIR/$nxt_obj:	$nxt_src $NXT_VERSION_H
 	\$(CC) -c \$(CFLAGS) \$(NXT_INCS) $NXT_PHP_INCLUDE \\
 	-DNXT_ZEND_SIGNAL_STARTUP=$NXT_ZEND_SIGNAL_STARTUP \\
 	$nxt_dep_flags \\