Rename 'readdrivestat' function on v3

Signed-off-by: Pablo Baeyens <pablo.baeyens@datadoghq.com>

v3/disk/disk_darwin_cgo.go

@@ -19,7 +19,7 @@ import (
 
 func IOCountersWithContext(ctx context.Context, names ...string) (map[string]IOCountersStat, error) {
 	var buf [C.NDRIVE]C.DriveStats
-	n, err := C.readdrivestat(&buf[0], C.int(len(buf)))
+	n, err := C.v3readdrivestat(&buf[0], C.int(len(buf)))
 	if err != nil {
 		return nil, err
 	}

v3/disk/iostat_darwin.c

@@ -16,7 +16,7 @@ static int getdrivestat(io_registry_entry_t d, DriveStats *stat);
 static int fillstat(io_registry_entry_t d, DriveStats *stat);
 
 int
-readdrivestat(DriveStats a[], int n)
+v3readdrivestat(DriveStats a[], int n)
 {
 	mach_port_t port;
 	CFMutableDictionaryRef match;

v3/disk/iostat_darwin.h

@@ -29,5 +29,4 @@ struct CPUStats {
 	natural_t idle;
 };
 
-extern int readdrivestat(DriveStats a[], int n);
+extern int v3readdrivestat(DriveStats a[], int n);
-extern int readcpustat(CPUStats *cpu);