fix make error for lacking namespace (#170)

diff --git a/src/gflags.h.in b/src/gflags.h.in
index 09daac3..3732636 100644
--- a/src/gflags.h.in
+++ b/src/gflags.h.in
@@ -581,6 +581,7 @@
 #define DEFINE_string(name, val, txt)                                       \
   namespace fLS {                                                           \
     using ::fLS::clstring;                                                  \
+    using ::fLS::StringFlagDestructor;                                      \
     static union { void* align; char s[sizeof(clstring)]; } s_##name[2];    \
     clstring* const FLAGS_no##name = ::fLS::                                \
                                    dont_pass0toDEFINE_string(s_##name[0].s, \