Switch the rest of our internal headers to #pragma once.

We've been using #pragma once for new internal files, but let's be more bold.

Bug: N/A
Test: builds
Change-Id: I7e2ee2730043bd884f9571cdbd8b524043030c07
diff --git a/libm/freebsd-compat.h b/libm/freebsd-compat.h
index ee41e45..0bcc40d 100644
--- a/libm/freebsd-compat.h
+++ b/libm/freebsd-compat.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _BIONIC_LIBM_FREEBSD_COMPAT_H_included
-#define _BIONIC_LIBM_FREEBSD_COMPAT_H_included
+#pragma once
 
 #include <float.h>
 
@@ -38,5 +37,3 @@
 
 // Similarly rename _scan_nan.
 #define _scan_nan __libm_scan_nan
-
-#endif