util/spkmodem-recv: fix getopt prototype

i use -pedantic and std=c90

also add the define

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/spkmodem_recv/spkmodem-recv.c

@@ -7,6 +7,8 @@
  * This version is heavily modified, re-written based on OpenBSD Kernel Source
  * File Style Guide (KNF); this change is Copyright 2023,2026 Leah Rowe. */
 
+#define _POSIX_SOURCE
+
 #ifdef __OpenBSD__
 #include <sys/param.h>
 #endif
@@ -25,6 +27,10 @@ static void err(int nvm_errval, const char *msg, ...);
 static const char * getnvmprogname(void);
 static void set_err_if_unset(int x);
 
+int getopt(int, char * const *, const char *);
+extern char *optarg;
+extern int optind, opterr, optopt;
+
 #if defined(__OpenBSD__) && defined(OpenBSD)
 #if OpenBSD >= 509
 #ifndef HAVE_PLEDGE