PNP: Mark the function pnp_build_option() as static in resource.c

This patch marks the function pnp_build_option() as static in resource.c
because it is not used outside this file.

Thus, it also eliminates the following warning in resource.c:
drivers/pnp/resource.c:34:20: warning: no previous prototype for ‘pnp_build_option’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>