libgsm installs headers in a subdirectory, use gsm.h from that subdirectory.

Approved by Diego at FOSDEM.

Originally committed as revision 21659 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2534,7 +2534,7 @@ enabled libdirac   && add_cflags $(pkg-config --cflags dirac) &&
                       require  libdirac libdirac_encoder/dirac_encoder.h dirac_encoder_init $(pkg-config --libs dirac)
 enabled libfaac    && require2 libfaac "stdint.h faac.h" faacEncGetVersion -lfaac
 enabled libfaad    && require2 libfaad faad.h faacDecOpen -lfaad
-enabled libgsm     && require  libgsm gsm.h gsm_create -lgsm
+enabled libgsm     && require  libgsm gsm/gsm.h gsm_create -lgsm
 enabled libmp3lame && require  libmp3lame lame/lame.h lame_init -lmp3lame -lm
 enabled libnut     && require  libnut libnut.h nut_demuxer_init -lnut
 enabled libopencore_amrnb  && require libopencore_amrnb opencore-amrnb/interf_dec.h Decoder_Interface_init -lopencore-amrnb -lm