Update src/stdlib_math.fypp

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: jalvesz

src/stdlib_math.fypp

@@ -557,7 +557,7 @@ contains
     end subroutine
 
     elemental subroutine swap_stt(lhs,rhs)
-        use stdlib_string_type
+        use stdlib_string_type, only: string_type
         type(string_type), intent(inout) :: lhs, rhs
         type(string_type) :: temp
         temp = lhs ; lhs = rhs ; rhs = temp