build: fix missing include

build: fix missing include

Fix a build failure:

virt-host-validate.c: In function 'main':
virt-host-validate.c:82:5: error: implicit declaration of function 'setlocale' [-Werror=implicit-function-declaration]
virt-host-validate.c:82:5: error: nested extern declaration of 'setlocale' [-Werror=nested-externs]
virt-host-validate.c:82:20: error: 'LC_ALL' undeclared (first use in this function)

* tools/virt-host-validate.c: Add <locale.h>.
* .gitignore: Ignore built executable.