package/mesa3d-headers: only check for conflicts when building

The configuration checks should only be done when building so that
make allyespackageconfig && make source works.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 509956da4cdba6a76214322b1ace979bbe6cd6a8)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mesa3d-headers/mesa3d-headers.mk

@@ -6,7 +6,7 @@
 
 # mesa3d-headers is inherently incompatible with mesa3d, so error out
 # if both are enabled.
-ifeq ($(BR2_PACKAGE_MESA3D)$(BR2_PACKAGE_MESA3D_HEADERS),yy)
+ifeq ($(BR_BUILDING)$(BR2_PACKAGE_MESA3D)$(BR2_PACKAGE_MESA3D_HEADERS),yyy)
 $(error mesa3d-headers enabled, but mesa3d enabled too)
 endif