Update gamma.hpp (#1236)

* Update gamma.hpp

Missing name for Lanczos parameter in gamma_imp_final

* Update gamma.hpp

* Change location of void and annotate

---------

Co-authored-by: Matt Borland <[email protected]>

Author: kenarab

Diff for: include/boost/math/special_functions/gamma.hpp

@@ -111,11 +111,13 @@ BOOST_MATH_GPU_ENABLED T sinpx(T z)
 // tgamma(z), with Lanczos support:
 //
 template <class T, class Policy, class Lanczos>
-BOOST_MATH_GPU_ENABLED T gamma_imp_final(T z, const Policy& pol, const Lanczos&)
+BOOST_MATH_GPU_ENABLED T gamma_imp_final(T z, const Policy& pol, const Lanczos& l)
 {
    BOOST_MATH_STD_USING
+   
+   (void)l; // Suppresses unused variable warning when BOOST_MATH_INSTRUMENT is not defined
 
-   T result = 1;
+   T result = 1;  
 
 #ifdef BOOST_MATH_INSTRUMENT
    static bool b = false;