modp_b64: Fix BUILD.gn to be able to build the library with GN am: 8a6dfb4ec3 am: b0bff9549b
am: c5eaf1bed0

* commit 'c5eaf1bed04c7c31a719ec77bf5af2eaa7463534':
  modp_b64: Fix BUILD.gn to be able to build the library with GN

Change-Id: I646a82ad63e4a33e21690e355b5c0556f5000d50