Clean up some obsolete NetBSD portability cruft.

Test: treehugger
Change-Id: I9197e6ebcaec6811ff5ee12e4d2e7a14c2881f64
diff --git a/libc/upstream-netbsd/android/include/extern.h b/libc/upstream-netbsd/android/include/extern.h
index 616becd..b8e6151 100644
--- a/libc/upstream-netbsd/android/include/extern.h
+++ b/libc/upstream-netbsd/android/include/extern.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _BIONIC_NETBSD_EXTERN_H_included
-#define _BIONIC_NETBSD_EXTERN_H_included
+#pragma once
 
 #include <sys/cdefs.h>
 
@@ -24,5 +23,3 @@
 const char* __strsignal(int, char*, size_t);
 
 __END_DECLS
-
-#endif