Fix multiple define error

Fix multiple define error

 This problem is met during build as ASAN.
 Add global variable '_rdp_log_dom' to extern keyword.
 The global variable in a header file, then this definition
 will go to every c file that includes this header, and
 will get multiple define error.

 Error log :
 [   79s] /usr/lib/gcc/armv7l-tizen-linux-gnueabi/9.2.0/../../../../armv7l-tizen-linux-gnueabi/bin/ld: .libs/module_la-e_mod_rdp.o:/home/abuild/rpmbuild/BUILD/e-mod-tizen-rdp-0.1.2/src/e_mod_rdp.h:26: multiple definition of `_rdp_log_dom'; .libs/module_la-e_mod_main.o:/home/abuild/rpmbuild/BUILD/e-mod-tizen-rdp-0.1.2/src/e_mod_rdp.h:26: first defined here
[   79s] /usr/lib/gcc/armv7l-tizen-linux-gnueabi/9.2.0/../../../../armv7l-tizen-linux-gnueabi/bin/ld: .libs/module_la-e_mod_rdp_conf.o:(.bss.__odr_asan._rdp_log_dom+0x0): multiple definition of `__odr_asan._rdp_log_dom'; .libs/module_la-e_mod_main.o:(.bss.__odr_asan._rdp_log_dom+0x0): first defined here

Change-Id: Iee56a971302625818745e390e4235f12f80cb5b6
Signed-off-by: Seungha Son <seungha.son@samsung.com>