qemu: Disable curl explicitly, its not in DEPENDS

qemu: Disable curl explicitly, its not in DEPENDS

There is an error when build meta-toolchain-sdk on Ubuntu 10.10:

| Installing NATIVESDK packages
| Processing task-sdk-host-nativesdk...
| Processing task-cross-canadian-mips...
| error: Failed dependencies:
|       libcurl.so.4 is needed by qemu-nativesdk-0.15.1-r5.i686
NOTE: package meta-toolchain-gmae-1.0-r7: task do_populate_sdk: Failed
ERROR: Task 21 (/buildarea2/lyang1/poky/meta/recipes-core/meta/meta-toolchain-gmae.bb, do_populate_sdk) failed with exit code '1'

This is because qemu would depend on curl if it happened to be
available at build time and could confuse the -native and -nativesdk
librbaries.

[YOCTO #2305]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>