gmp: move BBCLASSEXTEND = "native nativesdk" from gmp.inc into 6.1.0 recipe

It's unlikely that native builds of gmp 4.2.1 (ie the last LGPLv2
version) would ever be required (and given that recent versions of gcc
require gmp >= 4.3.2, native builds of gmp 4.2.1 are unlikely to work
very well). Restrict native and nativesdk builds to gmp 6.1.0 only.

  https://gcc.gnu.org/install/prerequisites.html

(From OE-Core rev: 307fc5b8bb071cc51cbcbec26bbedbac695f716a)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/gmp/gmp.inc

@@ -9,5 +9,3 @@ PACKAGECONFIG ??= ""
 PACKAGECONFIG[readline] = "--with-readline=yes,--with-readline=no,readline"
 
 ARM_INSTRUCTION_SET = "arm"
-
-BBCLASSEXTEND = "native nativesdk"

meta/recipes-support/gmp/gmp_6.1.0.bb

@@ -29,3 +29,5 @@ do_install_append_class-target() {
 }
 
 SSTATE_SCAN_FILES += "gmp.h"
+
+BBCLASSEXTEND = "native nativesdk"