Add a missing #include to a header.

This was masked by lucky #include ordering in the .cpp files and
uncovered when we moved to the canonical ordering because the primary
header was included first (yay!). Unfortunately, I can't build this
locally so took a build-bot iteration to find it.

llvm-svn: 304789

llvm/include/llvm/DebugInfo/PDB/DIA/DIAEnumSymbols.h

@@ -12,6 +12,7 @@
 
 #include "DIASupport.h"
 #include "llvm/DebugInfo/PDB/IPDBEnumChildren.h"
+#include "llvm/DebugInfo/PDB/PDBSymbol.h"
 
 namespace llvm {
 namespace pdb {