Correct force inline syntax for IAR compiler

Unlinke other compilers supported, the IAR compiler requires the
pre-processor extension to force inline a method to be placed before
the keyword `template` if the method is declared with one.

drivers/Ticker.h

@@ -83,8 +83,12 @@ class Ticker : public TimerEvent, private NonCopyable<Ticker> {
      *  @param func pointer to the function to be called
      *  @param t the time between calls in seconds
      */
-    template <typename F>
-    MBED_FORCEINLINE void attach(F &&func, float t)
+#if defined(__ICCARM__)
+    MBED_FORCEINLINE template <typename F>
+#else
+    template <typename F> MBED_FORCEINLINE
+#endif
+    void attach(F &&func, float t)
     {
         attach_us(std::forward<F>(func), t * 1000000.0f);
     }