libsysprof-capture: Add a missing string.h include

It’s needed for `strdup()` which is now used in a macro in this header.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

src/libsysprof-capture/sysprof-macros-internal.h

@@ -59,6 +59,8 @@
 
 #pragma once
 
+#include <string.h>
+
 #define sysprof_assert_not_reached() assert (false)
 
 #define SYSPROF_N_ELEMENTS(a) (sizeof (a) / sizeof (*a))