prevent spamming of thread local forward declarations in C/C++ output (#21955)

(cherry picked from commit f47b27d532)

compiler/cgen.nim

@@ -1266,10 +1266,10 @@ proc genVarPrototype(m: BModule, n: PNode) =
   if sym.owner.id != m.module.id:
     # else we already have the symbol generated!
     assert(sym.loc.r != nil)
+    incl(m.declaredThings, sym.id)
     if sfThread in sym.flags:
       declareThreadVar(m, sym, true)
     else:
-      incl(m.declaredThings, sym.id)
       if sym.kind in {skLet, skVar, skField, skForVar} and sym.alignment > 0:
         m.s[cfsVars].addf "NIM_ALIGN($1) ", [rope(sym.alignment)]
       m.s[cfsVars].add(if m.hcrOn: "static " else: "extern ")