gdk-pixbuf: unbreak gdk-pixbuf loaders.cache

Fix gdk-pixbuf broken by host package moved to /lib instead of /usr/lib
Regression occured following commit 19ba17ee (Globally replace
$(HOST_DIR)/usr/lib with $(HOST_DIR)/lib).

The host package loader.cache now refer to $(HOST_DIR)/lib, and the target
one should refer to /usr/lib.

Fix it by adjusting the sed invocation.

[Peter: extend commit message]
Signed-off-by: Pierre CROKAERT <pct@crookies.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>