php: fix build error for snmp extension

PHP requires that net-snmp be built with the ability to load MIB code,
so force it on when the extension is enabled.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/php/Config.ext

@@ -219,6 +219,7 @@ config BR2_PACKAGE_PHP_EXT_FTP
 config BR2_PACKAGE_PHP_EXT_SNMP
 	bool "SNMP"
 	select BR2_PACKAGE_NETSNMP
+	select BR2_PACKAGE_NETSNMP_ENABLE_MIBS
 	help
 	  SNMP support