glibc: fix version string for 2.21

Reported-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/glibc/Config.in

@@ -48,3 +48,4 @@ config BR2_GLIBC_VERSION_STRING
 	default "2.19-svnr25243" if BR2_EGLIBC_VERSION_2_19
 	default "2.19" if BR2_GLIBC_VERSION_2_19
 	default "2.20" if BR2_GLIBC_VERSION_2_20
+	default "2.21" if BR2_GLIBC_VERSION_2_21