libc: add errno include

This solves missing `__errno` symbol linker errors

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

litex/soc/software/libc/missing.c

@@ -5,6 +5,7 @@
  */
 
 #include <stddef.h>
+#include <errno.h>
 
 int getentropy(void *v, size_t s) {
     return -1;