assimp: Explicitly use nobranch=1 in SRC_URI

Branch 'assimp_5.0_release' is not present in repo.

Error:
assimp-5.0.1-r0 do_fetch: Fetcher failure: Unable to find revision 8f0c6b04b2257a520aaab38421b2e090204b69df in branch assimp_5.0_release even from upstream

Set nobranch=1, to fetch from v5.0.1 tag.

Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>