lcs: must include unistd.h for (s)size_t

ssize_t and size_t were taken for granted after including
some C++ header files. That's not guaranteed and breaks
on FC17 with g++ 4.7.

Probably the "pure C++" lcs code shouldn't use these types;
for now let's simply include unistd.h.

src/syncevo/lcs.h

@@ -25,6 +25,9 @@
 #include <list>
 #include <ostream>
 
+// for size_t and ssize_t
+#include <unistd.h>
+
 #include <syncevo/declarations.h>
 SE_BEGIN_CXX
 namespace LCS {