preproc: Fix macro definition lines being offset by 1

This is most noticeable in preprocessor messages involving the
definition location of a macro (redefine warning, argument mismatch
errors). The preprocessor_data linenum_ variable never starts at zero as
far as I can tell.

changelog

@@ -38,6 +38,8 @@ Version 1.13.1+dev:
      show/hide them
    * The WML preprocessor now writes warnings to stderr for macros redefined
      without #undef, to help detect unintentional name clashes.
+   * Fix macro definition line numbers being offset by 1 in WML preprocessor
+     messages involving macros.
  * Editor:
    * Added Category field and color sliders to the Edit Label panel.
  * Miscellaneous and bug fixes:

src/serialization/preprocessor.cpp

@@ -903,7 +903,7 @@ bool preprocessor_data::get_chunk()
 
 				buffer.erase(buffer.end() - 7, buffer.end());
 				(*target_.defines_)[symbol] = preproc_define(buffer, items, target_.textdomain_,
-					                       linenum + 1, target_.location_);
+					                       linenum, target_.location_);
 				LOG_PREPROC << "defining macro " << symbol << " (location " << get_location(target_.location_) << ")\n";
 			}
 		} else if (command == "ifdef") {