build: Move back to upstream edlib (#2357)

### Problem description
The current edlib submodule references a commit that's not present in
<https://github.com/blawrence-ont/edlib>. This throws off submodule
initialization and makes building an ImHex Flatpak using upstream Git
HEAD require some nasty Git bodges.

### Implementation description
Switching to <https://github.com/Martinsos/edlib> in `.gitmodules` is
all that is necessary, since
<https://github.com/Martinsos/edlib/commit/0ddc23ea06abd95ab6227442acb4c27bd7607b68>
properly exists.

### Screenshots
_This section was intentionally left blank._

### Additional things
For those curious, I'm currently working around this by manually
appending
```gitconfig
[url "https://github.com/Martinsos/edlib"]
	insteadOf = "https://github.com/blawrence-ont/edlib"
```
to
`${flatpak_builder_state}/git/https_github.com_blawrence-ont_edlib/config`
after `flatpak-builder` has cloned the repository and failed. I know
this is a brittle solution.

.gitmodules

@@ -24,7 +24,7 @@
     ignore = dirty
 [submodule "lib/third_party/edlib"]
 	path = lib/third_party/edlib
-	url = https://github.com/blawrence-ont/edlib
+	url = https://github.com/Martinsos/edlib
 	ignore = dirty
 [submodule "lib/third_party/lunasvg"]
 	path = lib/third_party/lunasvg