parser: provide typedefs for comparison_fn_t and __free_fn_t

The POSIX standard never defines the typedefs `comparison_fn_t` and
`__free_fn_t`, but they are provided by glibc and user in the parsing
code. Provide the typedefs ourselves to fix compiling on musl based
systems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>

parser/parser_symtab.c

@@ -25,6 +25,9 @@
 #include "immunix.h"
 #include "parser.h"
 
+typedef int (*comparison_fn_t)(const void *, const void *);
+typedef void (*__free_fn_t)(void *);
+
 enum var_type {
 	sd_boolean,
 	sd_set,