[InlineSpiller] Fix missing LiveRegMatrix declaration. NFC.

b25b51eb639d adds a reference to LiveRegMatrix in Spiller.h without
including the header, or adding a declaration.  That breaks modules
builds.

It looks like it might get reverted in #137801, but let's fix the
breakage in the meantime.

llvm/include/llvm/CodeGen/Spiller.h

@@ -20,6 +20,7 @@ class MachineFunctionPass;
 class VirtRegMap;
 class VirtRegAuxInfo;
 class LiveIntervals;
+class LiveRegMatrix;
 class LiveStacks;
 class MachineDominatorTree;
 class MachineBlockFrequencyInfo;