not warning on _g declaration in main file

Author: vbar

docker/.clang-tidy

@@ -2,3 +2,5 @@ Checks: bugprone-*,-bugprone-narrowing-conversions,-bugprone-signed-char-misuse,
 CheckOptions:
   - key: bugprone-unused-return-value.CheckedFunctions
     value: etxn_burden;fee_base;float_compare;float_divide;float_exponent;float_exponent_set;float_int;float_invert;float_mantissa;float_mantissa_set;float_mulratio;float_multiply;float_negate;float_one;float_set;float_sign;float_sign_set;float_sum;ledger_seq;otxn_burden;otxn_generation;otxn_type;slot_count;slot_float;slot_type;slot_size;sto_subarray;sto_subfield;sto_validate
+  - key: bugprone-reserved-identifier.AllowedIdentifiers
+    value: _g