package/libxmlrpc: add BR2_USE_MMU dependency

Build fails with:
server.c: In function 'ServerDaemonize':
server.c:1590:13: error: implicit declaration of function 'fork'
[-Wimplicit-function-declaration]
 1590 |     switch (fork()) {
      |             ^~~~

Hence, add BR2_USE_MMU to avoid building with toolchains without MMU
support.

Fixes:
https://autobuild.buildroot.org/results/10464159f2a9ff41ea0921a45ce12ecd0fa50ae7/

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[Julien: move BR2_USE_MMU up to fix check-package error]
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/libxmlrpc/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_LIBXMLRPC
 	bool "libxmlrpc"
 	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_LIBCURL
 	help
 	  XML-RPC is a quick-and-easy way to make procedure calls over
@@ -24,4 +25,5 @@ comment "cli tool needs a toolchain w/ dynamic library"
 endif # BR2_PACKAGE_LIBXMLRPC
 
 comment "libxmlrpc needs a toolchain w/ threads"
+	depends on BR2_USE_MMU
 	depends on !BR2_TOOLCHAIN_HAS_THREADS