add missing <cstring> include for strncpy, fixes musl

I/usr/include/libdbusmenu-glib-0.4 -flto -fdiagnostics-color=always -DNDEBUG -pipe -D_FILE_OFFSET_BITS=64 -std=c++17 -DHAVE_SWAY -DHAVE_LIBPULSE -DHAVE_DBUSMENU -D_FORTIFY_SOURCE=2 -mtune=generic -O2 -D_REENTRANT -pthread  -MD -MQ 'waybar@exe/src_modules_sway_ipc_client.cpp.o' -MF 'waybar@exe/src_modules_sway_ipc_client.cpp.o.d' -o 'waybar@exe/src_modules_sway_ipc_client.cpp.o' -c ../src/modules/sway/ipc/client.cpp
../src/modules/sway/ipc/client.cpp: In member function 'int waybar::modules::sway::Ipc::open(const string&) const':
../src/modules/sway/ipc/client.cpp:47:3: error: 'strncpy' was not declared in this scope
   strncpy(addr.sun_path, socketPath.c_str(), sizeof(addr.sun_path) - 1);
   ^~~~~~~
../src/modules/sway/ipc/client.cpp:47:3: note: 'strncpy' is defined in header '<cstring>'; did you forget to '#include <cstring>'?
../src/modules/sway/ipc/client.cpp:2:1:
+#include <cstring>

../src/modules/sway/ipc/client.cpp:47:3:
   strncpy(addr.sun_path, socketPath.c_str(), sizeof(addr.sun_path) - 1);
   ^~~~~~~
[36/44] Compiling C++ object 'waybar@exe/src_modules_custom.cpp.o'.
[37/44] Compiling C++ object 'waybar@exe/src_client.cpp.o'.
[38/44] Compiling C++ object 'waybar@exe/src_modules_cpu.cpp.o'.
ninja: build stopped: subcommand failed.

include/modules/sway/ipc/client.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <iostream>
+#include <cstring>
 #include <unistd.h>
 #include <sys/socket.h>
 #include <sys/un.h>