remove warnings about 'not a directory'

[bchamagne]

mkdir -p priv/c_dist
/usr/bin/gcc src/c/crypto/stdio_helpers.c src/c/crypto/ed25519.c -o priv/c_dist/libsodium_port -I src/c/crypto/stdio_helpers.h -lsodium
cc1: warning: src/c/crypto/stdio_helpers.h: not a directory
cc1: warning: src/c/crypto/stdio_helpers.h: not a directory
/usr/bin/gcc src/c/hypergeometric_distribution.c -o priv/c_dist/hypergeometric_distribution -lgmp
/usr/bin/gcc src/c/crypto/stdio_helpers.c src/c/crypto/tpm/lib.c src/c/crypto/tpm/port.c -o priv/c_dist/tpm_port -I src/c/crypto/stdio_helpers.h -I src/c/crypto/tpm/lib.h -ltss2-esys -ltss2-rc -ltss2-mu -lcrypto
cc1: warning: src/c/crypto/stdio_helpers.h: not a directory
cc1: warning: src/c/crypto/tpm/lib.h: not a directory
cc1: warning: src/c/crypto/stdio_helpers.h: not a directory
cc1: warning: src/c/crypto/tpm/lib.h: not a directory
cc1: warning: src/c/crypto/stdio_helpers.h: not a directory
cc1: warning: src/c/crypto/tpm/lib.h: not a directory
/usr/bin/gcc src/c/crypto/tpm/keygen.c src/c/crypto/tpm/lib.c -o priv/c_dist/tpm_keygen -I src/c/crypto/tpm/lib.h -ltss2-esys -ltss2-rc -ltss2-mu -lcrypto
cc1: warning: src/c/crypto/tpm/lib.h: not a directory
cc1: warning: src/c/crypto/tpm/lib.h: not a directory

to

/usr/bin/gcc src/c/crypto/stdio_helpers.c src/c/crypto/ed25519.c -o priv/c_dist/libsodium_port -I src/c/crypto -lsodium
/usr/bin/gcc src/c/hypergeometric_distribution.c -o priv/c_dist/hypergeometric_distribution -lgmp
/usr/bin/gcc src/c/crypto/stdio_helpers.c src/c/crypto/tpm/lib.c src/c/crypto/tpm/port.c -o priv/c_dist/tpm_port -I src/c/crypto -I src/c/crypto/tpm -ltss2-esys -ltss2-rc -ltss2-mu -lcrypto
/usr/bin/gcc src/c/crypto/tpm/keygen.c src/c/crypto/tpm/lib.c -o priv/c_dist/tpm_keygen -I src/c/crypto/tpm -ltss2-esys -ltss2-rc -ltss2-mu -lcrypto