bionic_auxv.h: fix #define conflict

Both libc/include/sys/auxv.h and libc/private/bionic_auxv.h
use _SYS_AUXV_H_ to see if a header file has already been included.
This prevents both of these files from being included at the same
time.

Fix this name conflict.

Change-Id: Ifaec88aa9779d784b95f8e75145117acf3d5cfc5
diff --git a/libc/private/bionic_auxv.h b/libc/private/bionic_auxv.h
index d3b0038..69d15b6 100644
--- a/libc/private/bionic_auxv.h
+++ b/libc/private/bionic_auxv.h
@@ -25,8 +25,8 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
-#ifndef _SYS_AUXV_H_
-#define _SYS_AUXV_H_
+#ifndef _PRIVATE_BIONIC_AUXV_H_
+#define _PRIVATE_BIONIC_AUXV_H_
 
 #include <elf.h>
 
@@ -36,4 +36,4 @@
 
 __END_DECLS
 
-#endif /* _SYS_AUXV_H_ */
+#endif /* _PRIVATE_BIONIC_AUXV_H_ */