Fix build (`ninja check` without running `ninja` first)

9218ff50 removed the BUILD.txt file, and as a subtle side-effect
libLLVMFrontendOpenACC wasn't a dependency of `ninja check` anymore.
However llvm-config requires all components to be built, and the
relevant test is broken when libLLVMFrontendOpenACC isn't built.

Unittest for libLLVMFrontendOpenACC are pending, but this addition
should fix some bots in the meantime.