Move header to modp_b64 subdir. libchromeos imports modp_b64 via modp_b64/modp_b64.h. Moving it into a subdirectory make the import cleaner. (cherry-pick from b1e6e2024f4b05e9747556c0937d6770da74ba13) Change-Id: Ib4b778bd103af1f75ea7ba5333e722a7699f91e7