Remove _FORTIFY_SOURCE in implementation files.

This was causing conflicting declarations for the library definitions of
common functions like sprintf(), snprintf(), and strchr().

Change-Id: I5daaa8a58183aa0d4d0fae8a7cb799671810f576

libc/bionic/strchr.cpp

@@ -27,6 +27,7 @@
  * SUCH DAMAGE.
  */
 
+#undef _FORTIFY_SOURCE
 #include <string.h>
 
 extern "C" char* strchr(const char* p, int ch) {

libc/stdio/snprintf.c

@@ -31,6 +31,7 @@
  * SUCH DAMAGE.
  */
 
+#undef _FORTIFY_SOURCE
 #include <limits.h>
 #include <stdio.h>
 #include <string.h>

libc/stdio/sprintf.c

@@ -31,6 +31,7 @@
  * SUCH DAMAGE.
  */
 
+#undef _FORTIFY_SOURCE
 #include <stdio.h>
 #include <string.h>
 #include <stdarg.h>