satgen: Move importCell out of the header.

This function has no hope of ever getting inlined anyway, and it speeds
up yosys compile time by 7%.

Makefile

@@ -600,7 +600,7 @@ $(eval $(call add_include_file,backends/cxxrtl/cxxrtl_vcd_capi.cc))
 $(eval $(call add_include_file,backends/cxxrtl/cxxrtl_vcd_capi.h))
 
 OBJS += kernel/driver.o kernel/register.o kernel/rtlil.o kernel/log.o kernel/calc.o kernel/yosys.o
-OBJS += kernel/cellaigs.o kernel/celledges.o
+OBJS += kernel/cellaigs.o kernel/celledges.o kernel/satgen.o
 
 kernel/log.o: CXXFLAGS += -DYOSYS_SRC='"$(YOSYS_SRC)"'
 kernel/yosys.o: CXXFLAGS += -DYOSYS_DATDIR='"$(DATDIR)"' -DYOSYS_PROGRAM_PREFIX='"$(PROGRAM_PREFIX)"'