Define the two debug hooks the Linux port never implemented.

PMAP/main.c is compiled against PMAP/platform.h, where PlatDebugInit() and
PlatDebugDeinit() are declared as real functions, but PMAP-linux only defines
the COM port and message helpers, so linking fails with undefined references.

diff --git a/PMAP-linux/platform-linux.c b/PMAP-linux/platform-linux.c
index b9a0bd7..6087bcc 100644
--- a/PMAP-linux/platform-linux.c
+++ b/PMAP-linux/platform-linux.c
@@ -100,6 +100,16 @@ void PlatShowMessageB(const char *format, ...)
 }
 
 
+// PlatDPrintf() is redirected to PlatShowEMessage() by platform.h, so there is
+// no debug output file to manage here.
+void PlatDebugInit(void)
+{
+}
+
+void PlatDebugDeinit(void)
+{
+}
+
 int pstricmp(const char *s1, const char *s2)
 {
     char s1char, s2char;