What are you doing to make this happen? You didn't read in /usr/include/X11/Xlib.h (perhaps while following definitions), and then accidentally change it, did you?

HEADERS +=
linuxkbhit.h

I might be mistaken, but I would have expected the first line to end with a \, i.e.

HEADERS += \
linuxkbhit.h

Similarly

SOURCES +=
linuxkbhit.cpp
main.cpp

ought be

SOURCES += \
linuxkbhit.cpp \
main.cpp

Is that relevant?

You have two LIBS += lines both adding -lX11. Probably no harm, but just commenting.