packages: nginx: fix PCRE symbol name and fix dependency

Fix PCRE config symbol name and let HTTP_REWRITE select it as it
depends on PCRE.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@36483 3c298f89-4303-0410-b956-a3cf2f4a3e73