fix(NODE-7395): do not export mech_oid symbols

As described in the ticket, this fixes mongosh's build on x64 macOS
with Node.js 24, even though we haven't fully investigated why
that is the case.

Making these variables `static` is a good idea anyway since they
are not meant to be used by other compilation units or loaded
from the kerberos addon.

Without this patch, we receive:

    [2026/01/15 12:18:50.271] ld: warning: alignment (2) of atom '__ZN13node_kerberos15spnego_mech_oidE' is too small and may result in unaligned pointers
    [2026/01/15 12:18:50.271] ld: warning: alignment (2) of atom '__ZN13node_kerberos13krb5_mech_oidE' is too small and may result in unaligned pointers
    [2026/01/15 12:18:50.946] ld: warning: pointer not aligned at address 0x10945A92E ('node_kerberos::krb5_mech_oid' + 4 from /private/tmp/m/boxednode/<REDACTED:notary_signing_key_name>/node-v24.12.0/out/Release/kerberos.a(kerberos_unix.o))
    [2026/01/15 12:18:50.946] ld: warning: pointer not aligned at address 0x10945A942 ('node_kerberos::spnego_mech_oid' + 4 from /private/tmp/m/boxednode/<REDACTED:notary_signing_key_name>/node-v24.12.0/out/Release/kerberos.a(kerberos_unix.o))
    [2026/01/15 12:18:51.115] ld: unaligned pointer(s) for architecture x86_64
    [2026/01/15 12:18:51.115] clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Author: addaleax

src/unix/kerberos_unix.cc

@@ -11,10 +11,10 @@ using namespace Napi;
 #define GSS_MECH_OID_SPNEGO 6
 
 static char krb5_mech_oid_bytes[] = "\x2a\x86\x48\x86\xf7\x12\x01\x02\x02";
-gss_OID_desc krb5_mech_oid = {9, &krb5_mech_oid_bytes};
+static gss_OID_desc krb5_mech_oid = {9, &krb5_mech_oid_bytes};
 
 static char spnego_mech_oid_bytes[] = "\x2b\x06\x01\x05\x05\x02";
-gss_OID_desc spnego_mech_oid = {6, &spnego_mech_oid_bytes};
+static gss_OID_desc spnego_mech_oid = {6, &spnego_mech_oid_bytes};
 
 /// KerberosClient
 void KerberosClient::Step(const CallbackInfo& info) {