[openmp] Fix missing include directory in omptest tool

Add missing `LIBOMP_INCLUDE_DIR` include directory to fix build
failures in omptest, as reported
in llvm#154786 (comment).
Thanks fo @jprotze for the suggested fix.

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

openmp/tools/omptest/CMakeLists.txt

@@ -22,6 +22,8 @@ endif()
 
 include(CMakePackageConfigHelpers)
 
+include_directories(${LIBOMP_INCLUDE_DIR})
+
 set(OMPTEST_HEADERS
   ./include/AssertMacros.h
   ./include/InternalEvent.h