build: increase file descriptor count limit for the build, some systems (e.g. Mac OS X default to 256, which is too little for some parallel builds)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34023 3c298f89-4303-0410-b956-a3cf2f4a3e73