Move systemd-sbsign to its own source subdirectory

It's already two files, and I expect that more will come. It's nicer to give
its own subdirectory to maintain consistent structure.

meson.build

@@ -2345,7 +2345,6 @@ subdir('src/backlight')
 subdir('src/battery-check')
 subdir('src/binfmt')
 subdir('src/bless-boot')
-subdir('src/boot')
 subdir('src/boot/efi')
 subdir('src/bootctl')
 subdir('src/busctl')
@@ -2411,6 +2410,7 @@ subdir('src/rfkill')
 subdir('src/rpm')
 subdir('src/run')
 subdir('src/run-generator')
+subdir('src/sbsign')
 subdir('src/shutdown')
 subdir('src/sleep')
 subdir('src/socket-activate')