build: package: install rsync even when package has no makedepends

When an APKBUILD has no makedepends, depends_build would be empty so we
would skip adding rsync when building with --src.

Fix this logic so we always add rsync even when there are no other
makedepends.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: !2581

pmb/build/_package.py

@@ -693,10 +693,13 @@ def packages(
         if depends_host:
             logging.info("*** Install host dependencies")
             pmb.chroot.apk.install(depends_host, hostchroot, build=False)
+
+        # If build with --src then we need rsync to copy in the source files.
+        if src:
+            depends_build.append("rsync")
+
         if depends_build:
             logging.info("*** Install build dependencies")
-            if src:
-                depends_build.append("rsync")
             pmb.chroot.apk.install(depends_build, buildchroot, build=False)
 
         # Build and finish up