Fix 'declaration shadows a local variable' warning

[popizdeh]

When compiling for AVX and AVX2 code expands to
`
// stbir__4_coeff_start

stbir__simdf8 tot0,c,cs;                      \
STBIR_SIMD_NO_UNROLL(decode);                 \
stbir__simdf8_load4b( cs, hc );               \
stbir__simdf8_0123to00112233( c, cs );        \
stbir__simdf8_mult_mem( tot0, c, decode );

// stbir__store_output

{ stbir__simdf t,c;                           \
stbir__simdf8_add4halves( t, stbir__if_simdf8_cast_to_simdf4(tot0), tot0 );    \
stbir__simdf_0123to2301( c, t );              \
stbir__simdf_add( t, t, c );                  \
stbir__simdf_store2( output, t );             \
horizontal_coefficients += coefficient_width; \
++horizontal_contributors;                    \
output += 2; }`

Declaration of c in the nested scope shadows previously declared variable coming from expansion of stbir__4_coeff_start. I'm not sure what the logic is behind variable names but I'm just using the next available letter.