Fix build on Cygwin
diff --git a/src/gb/gprs_ns_frgre.c b/src/gb/gprs_ns_frgre.c
index e557c7e..57cf65e 100644
--- a/src/gb/gprs_ns_frgre.c
+++ b/src/gb/gprs_ns_frgre.c
@@ -43,12 +43,16 @@
#define GRE_PTYPE_IPv4 0x0800
#define GRE_PTYPE_KAR 0x0000 /* keepalive response */
+#ifndef IPPROTO_GRE
+# define IPPROTO_GRE 47
+#endif
+
struct gre_hdr {
uint16_t flags;
uint16_t ptype;
} __attribute__ ((packed));
-#if defined(__FreeBSD__) || defined(__APPLE__)
+#if defined(__FreeBSD__) || defined(__APPLE__) || defined(__CYGWIN__)
/**
* On BSD the IPv4 struct is called struct ip and instead of iXX
* the members are called ip_XX. One could change this code to use