gen-introspect: Fix exit code for missing header files
gen-introspect: Fix exit code for missing header files

Fixes bug 633596.

tjol/vala-tjol,GNOME/vala,severinh/vala,flobrosch/vala-fixes,playya/vala,playya/vala,playya/vala,jessevdk/vala,GNOME/vala,frida/vala,kamanashisroy/aroop,severinh/vala,kamanashisroy/aroop,flobrosch/vala-fixes,frida/vala,tjol/vala-tjol,rvu95/vala,frida/vala,Distrotech/vala,tjol/vala-tjol,jessevdk/vala,GNOME/vala,rvu95/vala,jessevdk/vala,Distrotech/vala,rvu95/vala,kamanashisroy/aroop,frida/vala,Distrotech/vala,xdave/pvala,spidermonkdust/vala,GNOME/vala,flobrosch/vala-fixes,spidermonkdust/vala,severinh/vala,frida/vala,spidermonkdust/vala