esp32.ld: Add an entry for ets_write_char_uart

It is mentioned in ets_sys.h, but not actually PROVIDEd.

Merges https://github.com/espressif/esp-idf/pull/545

components/esp32/ld/esp32.rom.ld

@@ -1803,6 +1803,7 @@ PROVIDE ( ets_set_user_start = 0x4000687c );
 PROVIDE ( ets_unpack_flash_code = 0x40007018 );
 PROVIDE ( ets_unpack_flash_code_legacy = 0x4000694c );
 PROVIDE ( rom_main = 0x400076c4 );
+PROVIDE ( ets_write_char_uart = 0x40007cf8 );
 PROVIDE ( ets_install_putc1 = 0x40007d18 );
 PROVIDE ( ets_install_putc2 = 0x40007d38 );
 PROVIDE ( ets_install_uart_printf = 0x40007d28 );