fix build error when compiling against uclibc(++)

Fixes the following error:

In file included from ZNCString.cpp:10:0:
FileUtils.h: In static member function 'static CString CDir::GetCWD()':
FileUtils.h:246:36: error: 'getcwd' was not declared in this scope

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Uli Schlachter <psychon@znc.in>

FileUtils.h

@@ -16,6 +16,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/fcntl.h>
+#include <unistd.h>
 #include <vector>
 
 using std::vector;