devel/fb-adb: unbreak fetch after r400395

Mk/bsd.options.mk selection of generic options that predate _VARS support
includes DISTFILES but not MASTER_SITES unlike PATCHFILES + PATCH_SITES.

> android-ndk-r10e-linux-x86.bin doesn't seem to exist in /portdistfiles/.

> Attempting to fetch http://distcache.FreeBSD.org/ports-distfiles/android-ndk-r10e-linux-x86.bin

fetch: http://distcache.FreeBSD.org/ports-distfiles/android-ndk-r10e-linux-x86.bin: Not Found

Reported by: pkg-fallout