Fix dependency tree in Makefile to ensure schema is built *after* it's dropped when running in parallel

[ZimbiX]

Resolves https://github.com/paradoxxxzero/gnome-shell-system-monitor-applet/issues/786.

I barely have any experience with Makefiles, but managed to figure this one out pretty quickly.

Previously, when using -j4, it was showing an error on every second run:

➜ make -j4 install
  [remove      ]
/home/brendan/.local/share/gnome-shell/extensions/[email protected]
  [extension   ] OK
Reloading extensions does not work correctly and is no longer supported
  [reload      ] OK
  [remove      ] OK
  [translate   ] ar/ ca/ cs/ de/ es_ES/ es_MX/ fa/ fi/ fr/ hu/ it/ ja/
ko/ nl_NL/ pl/ pt/ pt_BR/ ro/ ru/ sk/ tr/ uk/ zh_CN/
  [translate   ] OK
cp: cannot stat
'[email protected]/schemas/gschemas.compiled': No
such file or directory
make: *** [Makefile:162: build] Error 1

Now it works consistently:

➜ make -j4 install
  [remove      ]
/home/brendan/.local/share/gnome-shell/extensions/[email protected]
  [gschemas    ] OK
  [extension   ] OK
Reloading extensions does not work correctly and is no longer supported
  [reload      ] OK
  [remove      ] OK
  [translate   ] ar/ ca/ cs/ de/ es_ES/ es_MX/ fa/ fi/ fr/ hu/ it/ ja/
ko/ nl_NL/ pl/ pt/ pt_BR/ ro/ ru/ sk/ tr/ uk/ zh_CN/
  [translate   ] OK
  [build       ] OK
  [install     ]
/home/brendan/.local/share/gnome-shell/extensions/[email protected]
Reloading extensions does not work correctly and is no longer supported
  [reload      ] OK
  [install     ] OK

[mgalgs]

LGTM. I don't know why the _drop-gschemas target even exists though since it's not used anywhere else... IMO the rm should just be part of the ./$(UUID)/schemas/gschemas.compiled recipe and _drop-gschemas should just be nuked...