Update keywords for extmem malloc

keywords.txt

 noexcept	LITERAL1
 static_assert	LITERAL1
 thread_local	LITERAL1
+size_t	LITERAL1
 
 # teensy specific functions
 elapsedMillis	LITERAL1
 MillisTimer	LITERAL1
 beginRepeating	KEYWORD2
 delayNanoseconds	KEYWORD2
+extmem_malloc	KEYWORD2
+extmem_free	KEYWORD2
+extmem_calloc	KEYWORD2
+extmem_realloc	KEYWORD2
 
 # removed by Arduino 1.0, now also removed from Teensyduino
 #BYTE	LITERAL1