uboot-tools: add missing !BR2_STATIC_LIBS dependency for FIT signature

BR2_PACKAGE_UBOOT_TOOLS_FIT_SUPPORT selects BR2_PACKAGE_DTC, but forgets
to depends on !BR2_STATIC_LIBS like BR2_PACKAGE_DTC does.

Fixes the following kconfig warning:

  arning: (BR2_PACKAGE_UBOOT_TOOLS_FIT_SUPPORT && BR2_PACKAGE_KVMTOOL && BR2_PACKAGE_UBOOT_TOOLS_MKIMAGE_FIT_SIGNATURE_SUPPORT) selects BR2_PACKAGE_DTC which has unmet direct dependencies (!BR2_STATIC_LIBS)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>