Include what you use in AArch64AsmBackend.cpp

AArch64AsmBackend.cpp was not using any APIs from AArch64.h, and was
only including it for transitive dependencies.  Doing so is problematic
from include-what-you-use perspective, but it is also a layering issue
(it creates a dependency cycle between the primary AArch64 target
library and the MCTargetDesc library).

llvm-svn: 361774