fix ubuntu18.04 build error

[funnywwh]

crtmpserver/sources/common/src/platform/baseplatform.cpp:72:7: error: unused variable ‘err’ [-Werror=unused-variable] int err = errno; ^~~ crtmpserver/sources/common/src/platform/baseplatform.cpp: In function ‘bool setFdCloseOnExec(int)’: github/crtmpserver/sources/common/src/platform/baseplatform.cpp:86:7: error: unused variable ‘err’ [-Werror=unused-variable] int err = errno;