Update version in php_version.h as well

The version there is automatically updated during `./configure`, but
not on Windows.

Author: cmb69

main/php_version.h

@@ -2,7 +2,7 @@
 /* edit configure.ac to change version number */
 #define PHP_MAJOR_VERSION 7
 #define PHP_MINOR_VERSION 4
-#define PHP_RELEASE_VERSION 20
+#define PHP_RELEASE_VERSION 21
 #define PHP_EXTRA_VERSION "-dev"
-#define PHP_VERSION "7.4.20-dev"
-#define PHP_VERSION_ID 70420
+#define PHP_VERSION "7.4.21-dev"
+#define PHP_VERSION_ID 70421