fix: resolve diag viewer globstr include

[sit-d]

Summary

• Point the GLFW diagnostic viewer at the repository's existing conio/globstr.inc helper with a relative include path.
• Keeps the documented one-file build from failing before shader assembly begins when only the fasm2 include directory is on INCLUDE.

Verification

• fasm2 -e 5 glfw/diag_viewer/diag_viewer.asm diag_viewer.exe assembled successfully in 4 passes.
• file diag_viewer.exe reported a PE32+ x86-64 Windows GUI executable.
• git diff --check HEAD -- glfw/diag_viewer/diag_viewer.asm passed.

Notes

• Runtime GLFW/OpenGL validation was not run here; this only verifies assembly of the diagnostic viewer source.