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/linker/linker_phdr.h b/linker/linker_phdr.h
index d6276ed..9b9ce94 100644
--- a/linker/linker_phdr.h
+++ b/linker/linker_phdr.h
@@ -25,8 +25,8 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
-#ifndef LINKER_PHDR_H
-#define LINKER_PHDR_H
+
+#pragma once
 
 /* Declarations related to the ELF program header table and segments.
  *
@@ -134,5 +134,3 @@
 
 const char* phdr_table_get_interpreter_name(const ElfW(Phdr) * phdr_table, size_t phdr_count,
                                             ElfW(Addr) load_bias);
-
-#endif /* LINKER_PHDR_H */