animation: add missing header for libc++ after 6a26d08 (#30)

In file included from src/animation/AnimatedVariable.cpp:1:
In file included from ./include/hyprutils/animation/AnimatedVariable.hpp:3:
./include/hyprutils/animation/AnimationConfig.hpp:17:60: error: implicit instantiation of undefined template 'std::basic_string<char>'
   17 |             std::string                                    internalBezier  = "";
      |                                                            ^
/usr/include/c++/v1/__fwd/string.h:43:28: note: template is declared here
   43 | class _LIBCPP_TEMPLATE_VIS basic_string;
      |                            ^
In file included from src/animation/AnimatedVariable.cpp:1:
In file included from ./include/hyprutils/animation/AnimatedVariable.hpp:3:
./include/hyprutils/animation/AnimationConfig.hpp:18:60: error: implicit instantiation of undefined template 'std::basic_string<char>'
   18 |             std::string                                    internalStyle   = "";
      |                                                            ^
/usr/include/c++/v1/__fwd/string.h:43:28: note: template is declared here
   43 | class _LIBCPP_TEMPLATE_VIS basic_string;
      |                            ^

include/hyprutils/animation/AnimationConfig.hpp

@@ -3,6 +3,7 @@
 #include "../memory/WeakPtr.hpp"
 #include "hyprutils/memory/WeakPtr.hpp"
 
+#include <string>
 #include <unordered_map>
 
 namespace Hyprutils {