libsrtp: openssl handling needs shared library support

Fixes:
http://autobuild.buildroot.net/results/ab8/ab80094080c18602a2adb266981f53ebe0647f63/
http://autobuild.buildroot.net/results/a77/a770318af841610e0c10f61cf61bdae298aa59a8/
http://autobuild.buildroot.net/results/ce2/ce27f5b81b74320f4d2efbbaa4754c23fb2feeab/
http://autobuild.buildroot.net/results/57f/57f54c6f00953e8b7da70f4467f38cc9dfca2380/

And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libsrtp/libsrtp.mk

@@ -25,7 +25,8 @@ endif
 # host-pkgconf to make sure pkg-config is installed.
 LIBSRTP_DEPENDENCIES = host-pkgconf
 
-ifeq ($(BR2_PACKAGE_OPENSSL),y)
+# openssl handling needs libdl support
+ifeq ($(BR2_PACKAGE_OPENSSL)x$(BR2_STATIC_LIBS),yx)
 LIBSRTP_DEPENDENCIES += openssl
 LIBSRTP_CONF_OPTS += --enable-openssl
 else