fish: fix postrm script interpreter

Add the missing shell interpreter line to the postrm script so apk can run the generated post-deinstall hook when fish is removed.

Fixes: #29348
Signed-off-by: Dharmik Parmar <dharmikparmar2004@yahoo.com>

Author: dhrm1k

utils/fish/Makefile

@@ -84,7 +84,8 @@ fi
 endef
 
 define Package/fish/postrm
-	rm -rf "$${IPKG_INSTROOT}/$(CONFIGURE_PREFIX)/share/fish/$(PKG_VERSION)"
+#!/bin/sh
+rm -rf "$${IPKG_INSTROOT}/$(CONFIGURE_PREFIX)/share/fish/$(PKG_VERSION)"
 endef
 
 $(eval $(call BuildPackage,fish))