build: fix stale package install stamps for unselected variants

When a package variant changes from =y to =m, its .install stamp file
was not being cleaned up, causing both variants to appear in the
package install list.

Fix by making compile depend on prepare-package-install, ensuring
the stamp file is always cleared. Only =y packages recreate it.

Signed-off-by: Felix Fietkau <[email protected]>

include/package.mk

@@ -395,7 +395,7 @@ prepare-package-install:
 $(PACKAGE_DIR):
 	mkdir -p $@
 
-compile:
+compile: prepare-package-install
 .install: .compile
 install: compile