Change the gmp download URL to https://gmplib.org/download

The FTP server sometimes gives the following message:

230-Your bandwidth usage is restricted
230-
230-GMP -- GNU Multiple Precision Arithmetic Library
230-
230->>>  This is an intensionally throttled ftp server.                 <<<
230->>>  For faster and less tamper-prone file download, please go to:  <<<
230->>>                                                                 <<<
230->>>                  https://gmplib.org/download/                   <<<
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 9524a7722bf165eab5f4578af3ddb72d3fb16700)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/gmp/rules.mak

 # GNU Multiple Precision Arithmetic
 
 GMP_VERSION := 6.0.0
-GMP_URL := ftp://ftp.gmplib.org/pub/gmp-$(GMP_VERSION)/gmp-$(GMP_VERSION).tar.bz2
+GMP_URL := https://gmplib.org/download/gmp-$(GMP_VERSION)/gmp-$(GMP_VERSION).tar.bz2
 
 $(TARBALLS)/gmp-$(GMP_VERSION).tar.bz2:
 	$(call download,$(GMP_URL))