Move struc FFTContext below SECTION_RODATA

Yasm creates an implicit unaligned text section if "struc" is used
outside of any section:
http://tortall.lighthouseapp.com/projects/78676-yasm/tickets/247

Since yasm only honors the "align" annotation on the first declaration
of a section, this implicit text section causes all text section
alignments to be ignored. Also fixes a yasm warning about it agnoring
alignment.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/fft_mmx.asm

@@ -36,6 +36,8 @@
 %define pointer resd
 %endif
 
+SECTION_RODATA
+
 struc FFTContext
     .nbits:    resd 1
     .reverse:  resd 1
@@ -47,8 +49,6 @@ struc FFTContext
     .tsin:     pointer 1
 endstruc
 
-SECTION_RODATA
-
 %define M_SQRT1_2 0.70710678118654752440
 %define M_COS_PI_1_8 0.923879532511287
 %define M_COS_PI_3_8 0.38268343236509