scripts/tags.sh: Ignore *.mod.c

CONFIG_MODVERSIONS=y results in a .mod.c for every compiled file in the
kernel. Issuing a 'make cscope' on a compiled kernel tree results in
the cscope files containing *.mod.c files.

[prarit@prarit linux]# make cscope
[prarit@prarit linux]# cat cscope.files | grep mod.c | wc -l
4807

These files are not useful for cscope and should be ignored. For example,

   #   line  filename / context / line
   1    105  arch/x86/kvm/kvm-intel.mod.c <<GLOBAL>>
             { 0x618911fc, __VMLINUX_SYMBOL_STR(numa_node) },
   2    508  drivers/block/mtip32xx/mtip32xx.h <<GLOBAL>>
             int numa_node;
   3     55  drivers/block/mtip32xx/mtip32xx.mod.c <<GLOBAL>>
             { 0x618911fc, __VMLINUX_SYMBOL_STR(numa_node) },
   4     37  drivers/cpufreq/acpi-cpufreq.mod.c <<GLOBAL>>
             { 0x618911fc, __VMLINUX_SYMBOL_STR(numa_node) },
   <snip>

Add an export to RCS_FIND_IGNORE so it can be used in scripts/tags.sh
and add explicitly ignore *.mod.c files.

Signed-off-by: Pra...