libpskc: fix implicit declaration with musl
global.c: In function 'pskc_free':
global.c:164:3: error: implicit declaration of function 'free' [-Werror=implicit-function-declaration]
164 | free (ptr);
| ^~~~
global.c:33:1: note: include '<stdlib.h>' or provide a declaration of 'free'
32 | # include <xmlsec/crypto.h>
+++ |+#include <stdlib.h>
33 | #endif
global.c:164:3: warning: incompatible implicit declaration of built-in function 'free' [-Wbuiltin-declaration-mismatch]
164 | free (ptr);
| ^~~~
global.c:164:3: note: include '<stdlib.h>' or provide a declaration of 'free'
global.c: In function '_pskc_debug':
global.c:205:7: warning: incompatible implicit declaration of built-in function 'free' [-Wbuiltin-declaration-mismatch]
205 | free (str);
| ^~~~
global.c:205:7: note: include '<stdlib.h>' or provide a declaration of 'free'