Drop [extern_c] annotation

The headers already add `extern "C"` blocks.

Author: Steelskin

stdlib/public/Platform/ucrt.modulemap

@@ -12,24 +12,24 @@
 
 // The following modules have to be standalone top-level modules to deal with
 // version 10.0.26100 of the Windows SDK.
-module _malloc [system] [extern_c] [no_undeclared_includes] {
+module _malloc [system] [no_undeclared_includes] {
   use corecrt
   header "malloc.h"
   export *
 }
 
-module _complex [system] [extern_c] [no_undeclared_includes] {
+module _complex [system] [no_undeclared_includes] {
   use corecrt
   header "complex.h"
   export *
 }
 
-module _stddef [system] [extern_c] [no_undeclared_includes] {
+module _stddef [system] [no_undeclared_includes] {
   header "stddef.h"
   export *
 }
 
-module _stdlib [system] [extern_c] [no_undeclared_includes] {
+module _stdlib [system] [no_undeclared_includes] {
   use corecrt
   header "stdlib.h"
   export *