client: util_vector: add include guards

cl_dll/util_vector.h

@@ -15,7 +15,8 @@
 //  Vector.h
 // A subset of the extdll.h in the project HL Entity DLL
 //
-
+#ifndef UTIL_VECTOR_H
+#define UTIL_VECTOR_H
 // Misc C-runtime library headers
 #include <stdio.h>
 #include <stdlib.h>
@@ -119,3 +120,4 @@ inline float DotProduct(const Vector& a, const Vector& b) { return(a.x*b.x+a.y*b
 inline Vector CrossProduct(const Vector& a, const Vector& b) { return Vector( a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.x*b.y - a.y*b.x ); }\
 
 #define vec3_t Vector
+#endif // UTIL_VECTOR_H