Fix mac builds due to unguarded-availability error

Just bump our required base version up, as developing Android already
effectively requires 10.13, and these tools won't be distributed to
end-user developers (like fastboot/adb/aapt/etc are)

external/bison/lib/gethrxtime.c:49:9: warning: 'clock_gettime' is only available on macOS 10.12 or newer [-Wunguarded-availability]

Test: build
Change-Id: I636007e942854274ee3705a48b4f793e1cd96b71
1 file changed