Fix Linux define

Fetched from upstream:
http://roy.marples.name/projects/dhcpcd/ci/2cabc3efee4c18ca?sbs=0

Signed-off-by: Baruch Siach <baruch@tkos.co.il>

Index: dhcpcd.h
==================================================================
--- /dhcpcd.h
+++ /dhcpcd.h
@@ -58,11 +58,11 @@
 
 struct interface {
 	struct dhcpcd_ctx *ctx;
 	TAILQ_ENTRY(interface) next;
 	char name[IF_NAMESIZE];
-#ifdef __linux
+#ifdef __linux__
 	char alias[IF_NAMESIZE];
 #endif
 	unsigned int index;
 	unsigned int flags;
 	sa_family_t family;