Fix compile error using asm

In function 'xPortGetCoreID':
error: expected ')' before ':' token
         :"=r"(id));

Merges https://github.com/espressif/esp-idf/pull/1965

components/freertos/include/freertos/portable.h

@@ -205,7 +205,7 @@ BaseType_t xPortInterruptedFromISRContext();
 /* Multi-core: get current core ID */
 static inline uint32_t IRAM_ATTR xPortGetCoreID() {
     int id;
-    asm (
+    __asm__ (
         "rsr.prid %0\n"
         " extui %0,%0,13,1"
         :"=r"(id));