[mlir][CAPI] Fix unused-but-set-variable warning in smt.c (NFC) (#188473)
llvm-project/mlir/test/CAPI/smt.c:37:21: warning: variable 'result' set
but not used [-Wunused-but-set-variable]
37 | MlirLogicalResult result = mlirTranslateModuleToSMTLIB(
| ^
1 warning generated.