ignore gcov output files

9 jobs for warnings in 3 minutes and 55 seconds
Status Job ID Name Coverage
  Test
passed #76191
VS2015
VS2015-32-bit

00:01:17

passed #76190
VS2015
VS2015-64-bit

00:01:20

passed #76189
VS2017
VS2017-32-bit

00:01:10

passed #76188
VS2015
VS2017-64-bit

00:01:18

passed #76185
Linux
clang-c++11

00:03:54

passed #76187
Linux
cppcheck

00:00:29

passed #76192
Linux
doc

00:00:16

passed #76184
Linux
gcc-c++11

00:02:11

passed #76186
Apple
macos-c++11

00:03:01