[CMake][OpenMP] Remove wrong backslash

outdir is defined in the line above, it will not exist in the install
command, so it should not be escaped.

(cherry picked from commit 50716ba)

Author: Flakebi

openmp/runtime/src/CMakeLists.txt

@@ -380,7 +380,7 @@ else()
       extend_path(outdir "${CMAKE_INSTALL_PREFIX}" "${OPENMP_INSTALL_LIBDIR}")
       install(CODE "execute_process(COMMAND \"\${CMAKE_COMMAND}\" -E create_symlink \"${LIBOMP_LIB_FILE}\"
         \"${alias}${LIBOMP_LIBRARY_SUFFIX}\" WORKING_DIRECTORY
-        \"\$ENV{DESTDIR}\${outdir}\")")
+        \"\$ENV{DESTDIR}${outdir}\")")
     endforeach()
   endif()
 endif()