app: revert `#pragma once` to macro guards in meson build.

Some change which happened with the meson port. From my research, this
`#pragma once` is not standard, though it is apparently implemented in
most common compilers.
Also apparently it is based on heuristic, hence it might happen that
file identity fails. Though unsure how often it would happen, if ever, I
don't find this very acceptable (and probably makes for hard-to-debug
bugs).

So let's revert to basic macro guards, stupid and no heuristic, which
don't ever fail and are very easy to read.

app/git-version.h.in

@@ -1,5 +1,8 @@
-#pragma once
+#ifndef __GIT_VERSION_H__
+#define __GIT_VERSION_H__
 
 #define GIMP_GIT_VERSION          "@GIMP_GIT_VERSION@"
 #define GIMP_GIT_VERSION_ABBREV   "@GIMP_GIT_VERSION_ABBREV@"
 #define GIMP_GIT_LAST_COMMIT_YEAR "@GIMP_GIT_LAST_COMMIT_YEAR@"
+
+#endif /* __GIT_VERSION_H__ */