tools: add pkg-config for preload_check_sign

The cflags and ldflags of preload_check_sign depend on the openssl
package thus pkg-config is needed to get the location where openssl
is installed.
This fix a potential build failure when openssl is not from the
distro and installed in a varied place.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
1 file changed