Update src/stdlib_linalg_iterative_solvers_pcg.fypp

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: jalvesz

src/stdlib_linalg_iterative_solvers_pcg.fypp

@@ -148,7 +148,7 @@ contains
         if(present(di))then
             di_ => di
         else 
-            allocate(di_(n),source=.false._1)
+            allocate(di_(n),source=.false._int8)
         end if
 
         ! workspace for the solver