termux-api.h: allow usage from c++

And prevent header from being included multiple times for no reason.

Author: Grimler91

termux-api.h

@@ -1,9 +1,22 @@
+#ifndef TERMUX_API_H
+#define TERMUX_API_H
+
 #include <stdlib.h>
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 _Noreturn void exec_am_broadcast(int, char**, char*, char*);
 _Noreturn void contact_plugin(int, char**, char*, char*);
 _Noreturn void exec_callback(int);
 void generate_uuid(char*);
 void* transmit_stdin_to_socket(void*);
 int transmit_socket_to_stdout(int);
 int run_api_command(int, char**);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif /* TERMUX_API_H */