test: sys/fcntl.h -> fcntl.h

POSIX.1-2008[1] says that the latter should be used, and all of our
supported platforms would seem to support this scheme, apparently even
Windows[2].

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
[2]: https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx

test/includes/pre/fcntl.h

@@ -1,4 +1,4 @@
-#include <sys/fcntl.h>
+#include <fcntl.h>
 
 static const mode_t kO_RDONLY = O_RDONLY;
 static const mode_t kO_WRONLY = O_WRONLY;

test/unit/os/fs_spec.lua

@@ -22,7 +22,7 @@ cimport('./src/nvim/main.h')
 cimport('./src/nvim/fileio.h')
 local fs = cimport('./src/nvim/os/os.h')
 cppimport('sys/stat.h')
-cppimport('sys/fcntl.h')
+cppimport('fcntl.h')
 cppimport('uv-errno.h')
 
 local buffer = ""