Fix linking breakage for rv_histogram

Since mpool is used in more places, it is necessary to link it; otherwise,
the following errors appear.

/usr/bin/ld: build/riscv.o: in function `block_map_clear':
riscv.c:(.text+0x68): undefined reference to `mpool_free'
/usr/bin/ld: riscv.c:(.text+0x7a): undefined reference to `mpool_free'
/usr/bin/ld: build/riscv.o: in function `rv_delete':
riscv.c:(.text+0x25d): undefined reference to `mpool_destroy'
/usr/bin/ld: riscv.c:(.text+0x269): undefined reference to `mpool_destroy'
/usr/bin/ld: build/riscv.o: in function `rv_create':
riscv.c:(.text+0x593): undefined reference to `mpool_create'
/usr/bin/ld: riscv.c:(.text+0x5aa): undefined reference to `mpool_create'
collect2: error: ld returned 1 exit status

mk/tools.mk

@@ -6,6 +6,7 @@ HIST_OBJS := \
 	map.o \
 	elf.o \
 	decode.o \
+	mpool.o \
 	rv_histogram.o
 
 HIST_OBJS := $(addprefix $(OUT)/, $(HIST_OBJS))