Musl libc fixes

Musl libc's <stdio.h> doesn't include <stdarg.h>, which cause
`va_start` and `va_end` end up being undefined symbols.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Leorize <alaviss@users.noreply.github.com>
Closes #6310

lib/libspl/include/assert.h

@@ -31,6 +31,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdarg.h>
 
 static inline int
 libspl_assert(const char *buf, const char *file, const char *func, int line)