FIX: {PREVIOUS_COLOUR} string control code was not activated for some reason. The logic for it is in place.

(cherry picked from commit 9d7c265fdca7884db26b3c2d0f856cc18a751c54)

Fixes: #41

src/table/strgen_tables.h

@@ -58,6 +58,7 @@ static const CmdStruct _cmd_structs[] = {
 	{"GRAY",              EmitSingleChar, SCC_GRAY,               0, -1, C_DONTCOUNT},
 	{"DKBLUE",            EmitSingleChar, SCC_DKBLUE,             0, -1, C_DONTCOUNT},
 	{"BLACK",             EmitSingleChar, SCC_BLACK,              0, -1, C_DONTCOUNT},
+    {"PREVIOUS_COLOUR",   EmitSingleChar, SCC_PREVIOUS_COLOUR,    0, -1, C_DONTCOUNT},
 
 	{"REV",               EmitSingleChar, SCC_REVISION,           0, -1, C_NONE}, // openttd revision string