fixed compile error for msvc

7 jobs for featureMergePlugin in 37 minutes and 17 seconds
Status Job ID Name Coverage
  Test
passed #24074
Apple
Mac-Cpp11

00:37:16

passed #24073
Qt551 VS2013
VS2013-Qt-5.5.1-x64

00:15:51

passed #24072
Qt56 VS2015
VS2015-Qt-5.6.0-x32

00:20:51

passed #24071
Qt56 VS2015
VS2015-Qt-5.6.0-x64

00:17:44

passed #24069
MultiThreads Linux
clang-c++11-Qt5

00:10:54

passed #24070
MultiThreads Linux
cppcheck

00:03:15

passed #24068
Linux
gcc-c++11-Qt5

00:23:05