Fix possible fail to check for dependencies

Few characters in filenames (a plus sign, a dot) can be interpreted
specially by grep. This can lead to the omission of missing package
dependency. For example if we would have "some.file.so" then it
matches also "some2file.so". -F switch off special meaning
of any character and -x match against whole line.

Signed-off-by: Sławomir Demeszko <s.demeszko@wireless-instruments.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44247 3c298f89-4303-0410-b956-a3cf2f4a3e73