osmesa: Fix Mingw build.

Build as

  scons platform=windows osmesa
diff --git a/src/mesa/drivers/osmesa/osmesa.c b/src/mesa/drivers/osmesa/osmesa.c
index 98f01fe..68d7114 100644
--- a/src/mesa/drivers/osmesa/osmesa.c
+++ b/src/mesa/drivers/osmesa/osmesa.c
@@ -515,11 +515,7 @@
    PACK_RGBA(p, color[0], color[1], color[2], color[3]);	\
 } while (0)
 
-#ifdef WIN32
-#include "..\swrast\s_linetemp.h"
-#else
 #include "swrast/s_linetemp.h"
-#endif
 
 
 
@@ -544,11 +540,7 @@
    }							\
 } while (0)
 
-#ifdef WIN32
-#include "..\swrast\s_linetemp.h"
-#else
 #include "swrast/s_linetemp.h"
-#endif
 
 
 
@@ -624,11 +616,7 @@
       span.z += span.zStep;					\
    }                                                            \
 }
-#ifdef WIN32
-#include "..\swrast\s_tritemp.h"
-#else
 #include "swrast/s_tritemp.h"
-#endif
 
 
 
@@ -656,11 +644,8 @@
       span.z += span.zStep;				\
    }                                                    \
 }
-#ifdef WIN32
-#include "..\swrast\s_tritemp.h"
-#else
+
 #include "swrast/s_tritemp.h"
-#endif