Fix MinGW build with CONFIG_EAP_TNC=y

MinGW does not use -ldl so do not add this unconditionally.

wpa_supplicant/Makefile

@@ -571,8 +571,10 @@ OBJS += ../src/eap_peer/tncc.o
 OBJS_h += ../src/eap_server/eap_tnc.o
 OBJS_h += ../src/eap_server/tncs.o
 NEED_BASE64=y
+ifndef CONFIG_NATIVE_WINDOWS
 LIBS += -ldl
 endif
+endif
 
 ifdef CONFIG_IEEE8021X_EAPOL
 # IEEE 802.1X/EAPOL state machines (e.g., for RADIUS authentication)