tools: bc: fix build on musl host

Fix declaration of getenv() in getopt.c

Backport from bc 1.08.2.

Signed-off-by: Shine <4c.fce2@proton.me>
Link: https://github.com/openwrt/openwrt/pull/21957
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

tools/bc/patches/001-getopt-declaration.patch

@@ -0,0 +1,11 @@
+--- a/lib/getopt.c
++++ b/lib/getopt.c
+@@ -197,7 +197,7 @@ static char *posixly_correct;
+    whose names are inconsistent.  */
+ 
+ #ifndef getenv
+-extern char *getenv ();
++extern char *getenv (const char *);
+ #endif
+ 
+ #endif /* not __GNU_LIBRARY__ */