parse: Don't #include <unistd.h>

It isn't needed and isn't present in non-posix environments like windows
with MSVC or mingw.

https://bugzilla.gnome.org/show_bug.cgi?id=774641

ges/ges-command-line-formatter.c

@@ -23,6 +23,7 @@
 #include "ges/ges-structured-interface.h"
 #include "ges-structure-parser.h"
 #include "ges-internal.h"
+#define YY_NO_UNISTD_H
 #include "ges-parse-lex.h"
 
 struct _GESCommandLineFormatterPrivate

ges/parse.l

@@ -9,6 +9,7 @@
 %option extra-type="GESStructureParser *"
 %option never-interactive
 %option noinput
+%option nounistd
 
 CLIP 		[ ]+\+clip[ ]+
 TEST_CLIP       [ ]+\+test-clip[ ]+