Fix integer(wp) non-standard kind for loop variables in bubble module

wp is a real kind parameter. integer(wp) is non-standard and may
produce wrong-sized integers on strict compilers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_bubbles_EE.fpp

@@ -74,7 +74,7 @@ contains
     subroutine s_comp_alpha_from_n(q_cons_vf)
         type(scalar_field), dimension(sys_size), intent(inout) :: q_cons_vf
         real(wp) :: nR3bar
-        integer(wp) :: i, j, k, l
+        integer :: i, j, k, l
 
         $:GPU_PARALLEL_LOOP(private='[i,j,k,l,nR3bar]', collapse=3)
         do l = 0, p