[flang] Fix typo in FlangConfig.cmake.in.

`find_package(Flang)` does not work as there is a missing `@` in the
FlangConfig.cmake.in file. This patch fixes the issue.

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D96484

Author: Leandro Vaz

flang/cmake/modules/FlangConfig.cmake.in

@@ -6,7 +6,7 @@ find_package(LLVM REQUIRED CONFIG
              HINTS "@FLANG_CONFIG_LLVM_CMAKE_DIR@")
 
 set(FLANG_EXPORTED_TARGETS "@FLANG_EXPORTS@")
-set(FLANG_CMAKE_DIR "FLANG_CONFIG_CMAKE_DIR@")
+set(FLANG_CMAKE_DIR "@FLANG_CONFIG_CMAKE_DIR@")
 set(FLANG_INCLUDE_DIRS "@FLANG_CONFIG_INCLUDE_DIRS@")
 
 # Provide all our library targets to users.