selftests: gpio: Find libmount with pkg-config if available

If pkg-config is available, use it to define the CFLAGS and
LDLIBS needed for libmount; else, use the current hard-coded
paths and options.

Using pkg-config is very helpful for cross-compilation
environments, and is sometimes readily available on developer
boxes to ensure we get the right compiler/linker options for
the given package.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>