pc-bios/dtb/meson: Prefer target name to be outfile, not infile

Makes this custom_target() usage consistent with other ones in QEMU.

Fixes: 6e0dc9d2a8 ("meson: compile bundled device trees")
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Link: https://lore.kernel.org/r/20250610204131.2862-3-shentey@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

pc-bios/dtb/meson.build

@@ -9,7 +9,7 @@ dtc = find_program('dtc', required: false)
 if dtc.found()
   foreach out : dtbs
     f = fs.replace_suffix(out, '.dts')
-    custom_target(f,
+    custom_target(out,
         build_by_default: have_system,
         input: files(f),
         output: out,