libnvme: add check-public-headers test and fix missing prototypes

[martin-belanger]

A few public function prototypes were missing from the installed public headers.

A symbol that appears in a version script but has no prototype in any installed header is technically callable by external code, but callers have no declaration to include — they must write their own prototype or fall back to dlsym().

This series fixes three such symbols and adds a test to prevent regressions.

Patch 1 — add missing prototypes for exported symbols

Three __public symbols were listed in version scripts (*.ld) but had no prototype in any installed header:

• libnvme_mi_submit_entry() — weak tracing hook in mi.c
• libnvme_mi_submit_exit() — weak tracing hook in mi.c
• libnvmf_context_set_fabrics_config() — setter in fabrics.c whose
  getter libnvmf_context_get_fabrics_config() was already declared

Prototypes added to mi.h and fabrics.h respectively.

Patch 2 — add check-public-headers test

Adds tools/check-public-headers.py, a companion to the existing check-public-symbols.py. It reads all *.ld version scripts, extracts their global: symbols, then scans each installed header for a matching prototype. Conditional headers (MI, fabrics) are silently skipped when absent, matching the build's own conditionality.

The script is registered as a meson test alongside check-public-symbols so it runs automatically under meson test.

[martin-belanger]

@igaw - I changed the script to take the list of headers and *.ld as input parameters as per your suggestion.

If you don't mind, I would like to address the other review comments as separate PRs. In other words, would it be possible to merge this PR, and I will work on fixes and submit another PR for your other comments (although for the MI I would prefer if someone else could handle it).