build: fix recipe line break in wrap_mirror fallback path

build: fix recipe line break in wrap_mirror fallback path

Commit 042996b46b ("build: stricter hash validation on download")
refactored the $(if) in wrap_mirror from a single line to multiple lines.
The else branch (used when MIRROR is empty) was moved to a new line without
a trailing backslash on the preceding line. This causes download methods
that use wrap_mirror (e.g. git) to fail when PKG_SOURCE_MIRROR is 0:

    SHELL= flock ...flock -c '  ...
    bash: -c: line 1: unexpected EOF while looking for matching `''

The fix is to add the missing \ to the else branch.

Signed-off-by: Karsten Sperling <ksperling@apple.com>