Here is a section of equates I found to be missing from the USER32.INC file:

; HT Mouse position codes
HTERROR = -2
HTTRANSPARENT = -1
HTNOWHERE = 0
HTCLIENT = 1
HTCAPTION = 2
HTSYSMENU = 3
HTGROWBOX = 4
HTSIZE = 4
HTMENU = 5
HTHSCROLL = 6
HTVSCROLL = 7
HTMINBUTTON = 8
HTMAXBUTTON = 9
HTLEFT = 10
HTRIGHT = 11
HTTOP = 12
HTTOPLEFT = 13
HTTOPRIGHT = 14
HTBOTTOM = 15
HTBOTTOMLEFT = 16
HTBOTTOMRIGHT = 17
HTBORDER = 18
HTREDUCE = 8
HTZOOM = 9
HTSIZEFIRST = 10
HTSIZELAST = 17
HTOBJECT = 19
HTCLOSE = 20
HTHELP = 21

These codes can be found in WinUser.h of the PlatformSDK