Don't use aliased function

getc is an alias to fgetc but aliases are only available in linklibs
Using getc here means that getc from stdcio.library was used and FILE
structure definitions are not compatible between posixc and stdcio
resulting in getc_unlocked function not working.

compiler/posixc/getc_unlocked.c

@@ -45,5 +45,5 @@
 
 ******************************************************************************/
 {
-    return getc(stream);
+    return fgetc(stream);
 }