package: Recommend virtual-locale-*, don't depend on it

The virtual-locale-* packages are provided by libc which may or
may not have a matching locale for any given recipes's provided locales.
Certainly, we shouldn't get a failure if the locale package isn't
available.

This patch therefore makes the dependency optional rather than
required, allowing the locale packages in question to install. This
resolves errors like:

	virtual-locale-eo is needed by bash-locale-eo-4.2-r5.i586
	virtual-locale-en+boldquot is needed by bash-locale-en+boldquot-4.2-r5.i586
	virtual-locale-en+quot is needed by bash-locale-en+quot-4.2-r5.i586

(From OE-Core rev: 2be67f95abaa7e8655a1ca8f4ffe66b3e099a650)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>