include libgen.h for getting prototype for basename()
Newer musl has remove prototype from string.h [1]
which renders a compile error with newer compilers like
clang 18+ about missing prototype.

[1] https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/third_party/openthread/tools/spi-hdlc-adapter/spi-hdlc-adapter.c
+++ b/third_party/openthread/tools/spi-hdlc-adapter/spi-hdlc-adapter.c
@@ -32,6 +32,7 @@
 #endif
 
 #include <assert.h>
+#include <libgen.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>