Use CROSS_COMPILE when setting PKG_CONFIG.

If CROSS_COMPILE is set on a Debian multiarch platform, then PKG_CONFIG
should be set the same way as the other cross compilation tools.

Change-Id: Id359a6bbdcbf8a136a0268a82301fc086a2adcfe
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>