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

* commit 'b0bff9549b39f3a511c14e35a5a976b11709375e':
  modp_b64: Fix BUILD.gn to be able to build the library with GN

Change-Id: I903776bdeb4ada7324d941ffc3b392f8a92d8dab