test/amg.c: include _hypre_parcsr_ls.h to fix build warning

[ColinIanKing]

The missing declaration of HYPRE_BoomerAMGGetCumNnzAP is throwing a warning when building with gcc, fix this by including _hypre_parcsr_ls.h

amg.c: In function 'main': amg.c:406:7: warning: implicit declaration of function 'HYPRE_BoomerAMGGetCumNnzAP'; did you mean 'HYPRE_BoomerAMGSetNumPaths'? [-Wimplicit-function-declaration] 406 | HYPRE_BoomerAMGGetCumNnzAP(pcg_precond, &cum_nnz_AP); | ^~~~~~~~~~~~~~~~~~~~~~~~~~ | HYPRE_BoomerAMGSetNumPaths