build: speed up kernel symbol export stripping

GNU grep has a high per-expression setup overhead when compiling regular
expressions. Use -F to force it to interpret the input as fixed strings,
which is much faster (fraction of a second instead of multiple minutes).

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39049 3c298f89-4303-0410-b956-a3cf2f4a3e73