Remove unused headers from hsStlUtils.h (and push them downstream where needed)

Also fix a couple of plString and header issues identified by Paradox on GCC/Clang

Sources/Plasma/PubUtilLib/plAvatar/plMatrixChannel.cpp

@@ -50,6 +50,8 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 //#include <hkmath/quaternion.h>
 //#include <hkmath/quaternion.h>
 
+#include <cmath>
+
 // singular
 #include "plMatrixChannel.h"
 
@@ -1015,4 +1017,4 @@ void plIK2Applicator::IApply(const plAGModifier *mod, double time)
 void plIK2Applicator::ISolve()
 {
 
-}
+}