[freerdp,api] add FREERDP_ENTRY_POINT

C requires prototypes or compilers will complain about them missing. Our
library entry points do not have such, therefore add the macro
FREERDP_ENTRY_POINT which declares the function prototype automatically
before the function.

channels/drdynvc/client/drdynvc_main.c

@@ -1970,7 +1970,8 @@ static int drdynvc_get_version(DrdynvcClientContext* context)
 /* drdynvc is always built-in */
 #define VirtualChannelEntryEx drdynvc_VirtualChannelEntryEx
 
-BOOL VCAPITYPE VirtualChannelEntryEx(PCHANNEL_ENTRY_POINTS_EX pEntryPoints, PVOID pInitHandle)
+FREERDP_ENTRY_POINT(BOOL VCAPITYPE VirtualChannelEntryEx(PCHANNEL_ENTRY_POINTS_EX pEntryPoints,
+                                                         PVOID pInitHandle))
 {
 	UINT rc;
 	drdynvcPlugin* drdynvc;