tools/thermal: tmon: fixed the 'make install' command

To install tmon we issue "make install" which produces bellow error.

root@odroidxu3:/usr/src/odroidxu3-4.y-testing/tools/thermal/tmon# make install
mkdir -p /usr/bin
install -m 755 -p "tmon" "/usr/bin/tmon"
mkdir -p /
install -m 644 -p "" "/"
install: cannot stat ‘’: No such file or directory
make: [install] Error 1 (ignored)

Signed-off-by: Anand Moon <linux.amoon@gmail.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>