libphobos: Add missing ControlState variable for AArch64

Fixes a typo that occurred during the splitting of the std.math module
into a package.

libphobos/ChangeLog:

	* src/std/math/hardware.d (FloatingPointControl.getControlState): Add
	missing ControlState variable for AArch64.

libphobos/src/std/math/hardware.d

@@ -912,6 +912,7 @@ private:
             }
             else version (AArch64)
             {
+                ControlState cont;
                 asm pure nothrow @nogc
                 {
                     "mrs %0, FPCR;" : "=r" (cont);