modp_b64: Generate pkgconfig

Generate a pkgconfig file for this project.

BUG=b:176847216
TEST=emerge-zork modp_b64

Change-Id: Ie1058750a56da435aba949ecd31ed15e47d20f56
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/external/modp_b64/+/2648693
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Commit-Queue: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>