refactor: rename argument to avoid C++ keyword conflict

The parameter `template` in format_call_external_argv() was renamed
to `template_fmt` to avoid collision with the `template` keyword in
C++. This improves compatibility when the header is included in
C++ code.

src/common.h

@@ -181,7 +181,7 @@ void get_file_paths_recursive(const char* directory, GSList** contents);
 gchar* get_random_string(int length);
 
 gboolean call_external(gchar** argv);
-gchar** format_call_external_argv(const char* template, const char* url, const char* filename);
+gchar** format_call_external_argv(const char* template_fmt, const char* url, const char* filename);
 
 gchar* unique_filename_from_url(const char* url, const char* path);
 gchar* get_expanded_path(const char* path);