o Reverse the inclusion chain from MD->MI to MI->MD by including

<machine/pmc_mdep.h> here.
o  Remove the #error directive. There's no union md_pm referenced
   on (as of yet) unsupported platforms and will not be if there
   are no MD extensions for a particular platform.

Further cleanups can be expected.

Ok'd: jkoshy@

sys/sys/pmc.h

@@ -29,6 +29,8 @@
 #ifndef _SYS_PMC_H_
 #define	_SYS_PMC_H_
 
+#include <machine/pmc_mdep.h>
+
 #define	PMC_MODULE_NAME		"hwpmc"
 #define	PMC_NAME_MAX		16 /* HW counter name size */
 #define	PMC_CLASS_MAX		4  /* #classes of PMCs in a CPU */
@@ -1115,10 +1117,6 @@ struct pmc {
 		} pm_amd;
 	} pm_md;
 
-#else
-
-#error	Unsupported PMC architecture.
-
 #endif
 };