[libc] Fix running 'stdbit.h' tests if file is never generated

Summary:
This test was added to test the generated header. Unfortunately this
doesn't work if the header is never generated. Add a check to make sure
the user has included it in the list of headers.

libc/test/include/CMakeLists.txt

@@ -17,7 +17,7 @@ add_libc_test(
 
 # stdbit_test only tests our generated stdbit.h, which is not generated in
 # overlay mode.
-if (LLVM_LIBC_FULL_BUILD)
+if(LLVM_LIBC_FULL_BUILD AND libc.include.stdbit IN_LIST TARGET_PUBLIC_HEADERS)
   add_libc_test(
     stdbit_test
     SUITE