add threadsafety.h to bitcoin-qt.pro

- to be able to see threadsafety.h in the Qt Creator IDE the file needs to
  be added to the HEADERS section

bitcoin-qt.pro

@@ -197,7 +197,8 @@ HEADERS += src/qt/bitcoingui.h \
     src/netbase.h \
     src/clientversion.h \
     src/txdb.h \
-    src/leveldb.h
+    src/leveldb.h \
+    src/threadsafety.h
 
 SOURCES += src/qt/bitcoin.cpp src/qt/bitcoingui.cpp \
     src/qt/transactiontablemodel.cpp \