Add missing SECTION .text marker in assembly file.

Fixes a crash on Windows when building with MSVC.

Change-Id: I124ac756a1be55d190fadda5fcc46d23b1445dbf

vp9/encoder/x86/vp9_subpel_variance.asm

@@ -61,6 +61,8 @@ bilin_filter_m_ssse3: times  8 db 16,  0
                       times  8 db  2, 14
                       times  8 db  1, 15
 
+SECTION .text
+
 ; int vp9_sub_pixel_varianceNxh(const uint8_t *src, ptrdiff_t src_stride,
 ;                               int x_offset, int y_offset,
 ;                               const uint8_t *dst, ptrdiff_t dst_stride,