Makefile.os2: -DSDL_USE_LIBICONV to fix linkage for GNU libiconv option

Makefile.os2

@@ -56,7 +56,7 @@ CFLAGS_DLL+= -bd
 # iconv:
 LIBICONV_LIB=iconv2.lib
 !ifeq LIBICONV 1
-CFLAGS_DLL+= -DHAVE_ICONV=1 -DHAVE_ICONV_H=1
+CFLAGS_DLL+= -DHAVE_ICONV=1 -DHAVE_ICONV_H=1 -DSDL_USE_LIBICONV
 LIBS+= $(ICONVLIB)
 !else
 LIBS+= libuls.lib libconv.lib