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/libc/seccomp/seccomp_bpfs.h b/libc/seccomp/seccomp_bpfs.h
index 8728c36..797dfc5 100644
--- a/libc/seccomp/seccomp_bpfs.h
+++ b/libc/seccomp/seccomp_bpfs.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef SECCOMP_BPFS_H
-#define SECCOMP_BPFS_H
+#pragma once
 
 #include <stddef.h>
 #include <linux/seccomp.h>
@@ -61,5 +60,3 @@
 extern const size_t mips64_system_filter_size;
 extern const struct sock_filter mips64_global_filter[];
 extern const size_t mips64_global_filter_size;
-
-#endif