Follow GCC recommendations for getting rid of it.

MingW/GCC (11.3, 12.x) complains with:
/home/eric/work/wine/dlls/d3dx9_36/tests/effect.c: In function 'test_effect_parameter_value':
/home/eric/work/wine/dlls/d3dx9_36/tests/effect.c:1838:71: warning: expression does not compute the number of elements in this array; element type is 'DWORD' {aka 'long unsigned int'}, not 'D3DXMATRIX' {aka 'struct _D3DMATRIX'} [-Wsizeof-array-div]
 1838 |             const D3DXMATRIX *matrix_pointer_array[sizeof(input_value)/sizeof(D3DXMATRIX)];
      |                                                                       ^
/home/eric/work/wine/dlls/d3dx9_36/tests/effect.c:1838:71: note: add parentheses around the second 'sizeof' to silence this warning
/home/eric/work/wine/dlls/d3dx9_36/tests/effect.c:1835:19: note: array 'input_value' declared here
 1835 |             DWORD input_value[EFFECT_PARAMETER_VALUE_ARRAY_SIZE];
      |                   ^~~~~~~~~~~
Signed-off-by: Eric Pouech <eric.pouech@gmail.com>