BCatalog: declare GetString as format_printf function

This tells the compiler that this function takes a format string as a
parameter, and returns a "similar" string with the same formatting
operations. This allows the format string to "propagate" to prinf-file
functions (including BString::SetToFormat) and the format string to be
actually checked with the arguments passed to that function.

Without this, all translated strings were not checked to match with
their arguments.

Change-Id: I5c3c5cbfe7dfede9a6f45cad47a7524f9138fac0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7663
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Haiku-Format: Haiku-format Bot <no-reply+haikuformatbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

headers/os/locale/Catalog.h

@@ -31,7 +31,8 @@ public:
 
 			const char*			GetString(const char* string,
 									const char* context = NULL,
-									const char* comment = NULL);
+									const char* comment = NULL)
+									__attribute__((format_arg(2)));
 			const char*			GetString(uint32 id);
 
 			status_t			GetData(const char* name, BMessage* msg);