package/mkpaswd: define extract step to allow patching

As recently discussed on the list:
http://lists.busybox.net/pipermail/buildroot/2021-April/308373.html

Packages like mkpasswd where the source code is included in the Buildroot
tree, currently doesn't use an extract step (which also skips the patch
step), so they cannot be patched by a patch in BR2_GLOBAL_PATCH_DIR (or the
pkgdir), which is a bit confusing.

As a fix, add a simple extract step that copies the source files to the
build directory, so the patch step gets run as well.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit a052e6e88342faa71b67ca3aff32c655d96e8ef4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mkpasswd/mkpasswd.mk

@@ -9,9 +9,13 @@
 # at revision 5a0f08500fa51608b6d3b73ee338be38c692eadb
 HOST_MKPASSWD_LICENSE = GPL-2.0+
 
+define HOST_MKPASSWD_EXTRACT_CMDS
+	cp $(HOST_MKPASSWD_PKGDIR)/*.c $(HOST_MKPASSWD_PKGDIR)/*.h $(@D)
+endef
+
 define HOST_MKPASSWD_BUILD_CMDS
 	$(HOSTCC) $(HOST_CFLAGS) $(HOST_LDFLAGS) \
-		package/mkpasswd/mkpasswd.c package/mkpasswd/utils.c \
+		$(@D)/mkpasswd.c $(@D)/utils.c \
 		-o $(@D)/mkpasswd -lcrypt
 endef