AP_Common: missing: Add definition for strndupa

`strndupa` is only available when using the GNU GCC suite.
With this definition is possible to use the MUSL compiler.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_Common/missing/string.h

@@ -0,0 +1,15 @@
+#include_next <string.h>
+
+// Necessary for toolchains that does not provide `strndupa`, such as musl.
+#if !defined(HAVE_DECL_STRNDUPA) && !defined(strndupa)
+// The last value of the GCC extension "statement exprs" will be
+// evaluated and returned, E.g: `#define foo(n) ({ n; })` is equivalent for
+// `auto foo(auto n) { return n; }`
+#define strndupa(old_string, len)                                     \
+({                                                                    \
+    const size_t string_len = strnlen(old_string, len);               \
+    char *new_string = static_cast<char*>(alloca(string_len + 1));    \
+    new_string[string_len] = '\0';                                    \
+    static_cast<char*>(memcpy(new_string, old_string, len));          \
+})
+#endif