Add missing dollar # OpenMP directive. Closes #840. (#841)

Author: ivan-pi

Diff for: src/fpm_environment.f90

@@ -41,7 +41,7 @@ integer function get_os_type() result(r)
         logical           :: file_exists
         logical, save     :: first_run = .true.
         integer, save     :: ret = OS_UNKNOWN
-        !omp threadprivate(ret, first_run)
+        !$omp threadprivate(ret, first_run)
 
         if (.not. first_run) then
             r = ret