[RV64_DYNAREC] Fix compilation error in _helper.c

Fix compilation error in RV64 dynarec helper function

Encountered a compilation failure with gcc 10.2.0 for RISC-V
when trying to declare a variable within a switch case
without enclosing it in braces.
This change fixes the issue by adding curly braces around the code block
inside the case statement, which is necessary for older versions of GCC
that require a declaration to be part of a block.

This resolves the error:
"error: a label can only be part of a statement and a declaration is not a statement"

Now, variable declarations inside switch cases are enclosed in braces
to ensure compatibility with older compilers and maintain consistent coding style.

src/dynarec/rv64/dynarec_rv64_helper.c

@@ -2694,11 +2694,12 @@ void vector_loadmask(dynarec_rv64_t* dyn, int ninst, int vreg, uint64_t imm, int
                     ADDI(s1, xZR, 1);
                     VMV_S_X(vreg, s1);
                     return;
-                case 2:
+                case 2: {
                     int scratch = fpu_get_scratch(dyn);
                     VMV_V_I(scratch, 1);
                     VSLIDE1UP_VX(vreg, scratch, xZR, VECTOR_UNMASKED);
                     return;
+                }
                 case 3:
                     VMV_V_I(vreg, 1);
                     return;