Fix build (ninja check without running ninja first)

9218ff5 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.

Author: joker-eph

llvm/unittests/Frontend/CMakeLists.txt

@@ -2,6 +2,7 @@ set(LLVM_LINK_COMPONENTS
   Analysis
   Core
   FrontendOpenMP
+  FrontendOpenACC
   Support
   Passes
   TransformUtils