libstore: add missing #include on darwin

optimise-store.cc used std::regex on darwin, but forgot to include the
header. This probably compiled due to the precompiled headers file, but
it caused errors in the editor.

Change-Id: I23297c08cb66d44e4d4f303560f46e4adc7d5a43

src/libstore/optimise-store.cc

@@ -9,6 +9,9 @@
 #include <unistd.h>
 #include <errno.h>
 
+#if __APPLE__
+#include <regex>
+#endif
 
 namespace nix {