Makefile.am: fixed missing includes for make dist targets

Due to missing includes, make dist archives could not be built.

Signed-off-by: Alexander Rumyantsev <cetygamer@gmail.com>

src/target/riscv/Makefile.am

@@ -7,10 +7,13 @@ noinst_LTLIBRARIES += %D%/libriscv.la
        %D%/debug_defines.h \
        %D%/debug_reg_printer.h \
        %D%/encoding.h \
+       %D%/field_helpers.h \
        %D%/gdb_regs.h \
        %D%/opcodes.h \
        %D%/program.h \
        %D%/riscv.h \
+       %D%/riscv_reg_impl.h \
+       %D%/riscv_reg.h \
        %D%/riscv-011.h \
        %D%/riscv-011_reg.h \
        %D%/riscv-013.h \