Fix conftest probes on kernels with allocation profiling

[mornepousse]

v1.15.0 fails to build on kernels with CONFIG_MEM_ALLOC_PROFILING=y:

include/linux/acpi.h:69:50: error: macro 'kzalloc_obj' requires 2 arguments, but only 1 given

That's evdi's fallback kzalloc_obj from evdi_debug.h shadowing the kernel's
own macro, on a kernel that provides it. The support is in place
(EVDI_HAVE_KZALLOC_OBJ); detection is what fails.

conftest.sh compiles probes with $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -DMODULE, but KBUILD_MODNAME is a
per-object define kbuild adds outside KBUILD_CFLAGS, so probes never see it.
Since the probes define MODULE, and with allocation profiling on, every
allocator expands through alloc_hooks → DEFINE_ALLOC_TAG → CT_MODULE_NAME,
which codetag.h defines as KBUILD_MODNAME under #ifdef MODULE:

include/linux/codetag.h:64:24: error: 'KBUILD_MODNAME' undeclared

Any probe touching an allocator then fails regardless of whether the API
exists, and evdi_detect.h reports EVDI_HAVE_KZALLOC_OBJ unset on a kernel
that has it. Kernels without allocation profiling are unaffected —
DEFINE_ALLOC_TAG is empty there — which likely explains why CI stayed green.

Giving the probes a module name is enough:

 CFLAGS=$(requote "$@")
+CFLAGS="$CFLAGS '-DKBUILD_MODNAME=\"conftest\"' '-DKBUILD_BASENAME=\"conftest\"'"

This is not tied to one kernel release: I reproduced the failure and the fix on
both Linux 7.1.8 and 7.2.0 (CachyOS x86-64-v3, CONFIG_MEM_ALLOC_PROFILING=y,
gcc 15.3). Unpatched, both fail at acpi.h:69; patched, both build evdi.ko.
Compiling the probe body standalone shows the same split — it fails only with
-DMODULE and no KBUILD_MODNAME, and a probe without an allocation compiles
either way.

v1.14.x is unaffected: conftest.sh arrived with 490e1e8, released in v1.15.0.

(Disclosure: diagnosed and tested with Claude Code — Claude Fable 5 for the
first analysis, Claude Opus 5 for the verification and this text. I reviewed
the reasoning and ran the builds.)